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# HD 68808 (AH Velorum)

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 Extended envelopes around Galactic Cepheids. I. ℓ Carinae from near and mid-infrared interferometry with the VLTIWe present the results of long-baseline interferometric observations ofthe bright southern Cepheid ℓ Carinae in the infrared N (8-13 μm)and K (2.0-2.4 μm) bands, using the MIDI and VINCI instruments of theVLT Interferometer. We resolve in the N band a large circumstellarenvelope (CSE) that we model with a Gaussian of 3 Rstar(≈500 Rȯ ≈ 2-3 AU) half width at half maximum. Thesignature of this envelope is also detected in our K band data as adeviation from a single limb darkened disk visibility function. Thesuperimposition of a Gaussian CSE on the limb darkened disk model of theCepheid star results in a significantly better fit of our VINCI data.The extracted CSE parameters in the K band are a half width at halfmaximum of 2 Rstar, comparable to the N band model, and atotal brightness of 4% of the stellar photosphere. A possibility is thatthis CSE is linked to the relatively large mass loss rate of ℓ Car.Though its physical nature cannot be determined from our data, wediscuss an analogy with the molecular envelopes of RV Tauri, redsupergiants and Miras. Mean Angular Diameters and Angular Diameter Amplitudes of Bright CepheidsWe predict mean angular diameters and amplitudes of angular diametervariations for all monoperiodic PopulationI Cepheids brighter than=8.0 mag. The catalog is intended to aid selecting mostpromising Cepheid targets for future interferometric observations. CHARM2: An updated Catalog of High Angular Resolution MeasurementsWe present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773 J - K DENIS photometry of a VLTI-selected sample of bright southern starsWe present a photometric survey of bright southern stars carried outusing the DENIS instrument equipped with attenuating filters. Theobservations were carried out not using the survey mode of DENIS, butwith individual target pointings. This project was stimulated by theneed to obtain near-infrared photometry of stars to be used in earlycommissioning observations of the ESO Very Large TelescopeInterferometer, and in particular to establish a network of brightcalibrator sources.We stress that near-infrared photometry is peculiarly lacking for manybright stars. These stars are saturated in 2MASS as well as in regularDENIS observations. The only other observations available for brightinfrared stars are those of the Two Micron Sky Survey dating from overthirty years ago. These were restricted to declinations above≈-30°, and thus cover only about half of the sky accessible fromthe VLTI site.We note that the final 2MASS data release includes photometry of brightstars, obtained by means of point-spread function fitting. However, thismethod only achieves about 30% accuracy, which is not sufficient formost applications.In this work, we present photometry for over 600 stars, each with atleast one and up to eight measurements, in the J and K filters. Typicalaccuracy is at the level of 0\fm05 and 0\fm04 in the J and K_s bands,respectively.Based on observations collected at the European Southern Observatory, LaSilla.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/413/1037 Fundamental Parameters of Cepheids. V. Additional Photometry and Radial Velocity Data for Southern CepheidsI present photometric and radial velocity data for Galactic Cepheids,most of them being in the southern hemisphere. There are 1250 Genevaseven-color photometric measurements for 62 Cepheids, the averageuncertainty per measurement is better than 0.01 mag. A total of 832velocity measurements have been obtained with the CORAVEL radialvelocity spectrograph for 46 Cepheids. The average accuracy of theradial velocity data is 0.38 km s-1. There are 33 stars withboth photometry and radial velocity data. I discuss the possiblebinarity or period change that these new data reveal. I also presentreddenings for all Cepheids with photometry. The data are availableelectronically. Based on observations obtained at the European SouthernObservatory, La Silla. On the Absolute Calibration of the Cepheid Distance Scale Using Hipparcos ParallaxesThe fundamental Hipparcos parallaxes (HIPP) of 219 Cepheids are used forthe absolute calibration of the Galactic distance scale sampled by amodern Baade-Wesselink (BW) distance indicator, which reliably accountsfor pulsation and thermal properties of Cepheid variable stars. Notablywe map thermal properties into the Johnson-Cousins color (V-I). The BWrealization is found to be much less affected than previously adoptedoptical luminosity laws by intrinsic scatter and systematic errors inrepresenting individual Cepheid distances and thus is best suited for acalibration of the galactic distance scale using the fundamentalHipparcos parallaxes (HIPP). Comparisons between the actual Hipparcoscalibration and three independent ground-based calibrations of the sameBW distance scale show very close agreement at the 0.04 mag level, i.e.,at the 1 σ level of the absolute accuracy claimed for BWrealizations, although the Hipparcos calibration is affected by anuncertainty of +/-0.10 mag due to propagation of parallax errors alone.Comparisons include the zero-age main-sequence calibration by Cepheidsin clusters (Pleiades distance modulus at 5.57 mag), the calibration bypulsation parallaxes of Cepheids, and the calibration by updated modelcalculations of synthetic stellar spectra of Cepheids. Notably, theresulting galactic distance scale is found to be ~0.1 mag shorter thanthe value obtained in the original calibration of Feast & Catchpole.The implications of the actual calibration on the Cepheid-based distanceto the Large Magellanic Cloud (LMC) and the extragalactic distance scaleare briefly discussed. A true distance modulus of 18.59+/-0.04 mag ispresently achieved. Evidence from Hipparcos-based calibrations bydifferent methods strongly supports the actual upward revision of0.09+/-0.04 mag for the LMC distance of 18.50 mag adopted in the HubbleSpace Telescope Key Project program, corresponding to no more than a 5%decrease in the value of the Hubble constant. CHARM: A Catalog of High Angular Resolution MeasurementsThe Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/386/492, and from theauthors on CD-Rom. Photoelectric Observations of Southern Cepheids in 2001A total of 2097 photometric observations in the BVIc systemare presented for 117 Cepheids located in the southern hemisphere. Themain purpose of the photometry is to provide new epochs of maximumbrightness for studying Cepheid period changes, as well as to establishcurrent light elements for the Cepheids. Spectroscopic investigations of classical Cepheids and main-sequence stars in galactic open clusters and associations. I. Association Cas OB2 and the small-amplitude Cepheid SU CassiopeaeThe small-amplitude Cepheid SU Cas and four membersof the association Cas OB2 (HD 16893, HD17327a and b, HD 17443) were investigated,using high-resolution CCD spectra. The following results were obtained:1) All these objects have the same metallicity values, close to that ofthe Sun; 2) Elemental abundance indicates that SU Cas is a post firstdredge-up star with an age from 1 108 to 1.45 108yr, and it is not crossing the Cepheid instability strip for the firsttime. The mean value of log g = 2.35 corresponds to pulsations in thefundamental tone, although errors in gravity estimations provideovertone pulsations. The questions about its pulsational mode andmembership in Cas OB2 remained open; 3) HD17327a is a slowly rotating HgMn-star with the highest heliumcontent among such objects, while HD 16893 also has a manganeseoverabundance and might be classified as an Am-star; 4) HD17327b and HD 17443 are rapidly rotating main-sequence stars,while HD 17443 has a helium content comparable with that of the Sun. Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statisticsThe Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521 Galactic Cepheids. Catalogue of light-curve parameters and distancesWe report a new version of the catalogue of distances and light-curveparameters for Galactic classical Cepheids. The catalogue listsamplitudes, magnitudes at maximum light, and intensity means for 455stars in BVRI filters of the Johnson system and (RI)_C filters of theCron-Cousins system. The distances are based on our new multicolour setof PL relations and on our Cepheid-based solution for interstellarextinction law parameters and are referred to an LMC distance modulus of18.25. The catalogue is only available in electronic form at the CDS viaanonymous ftp (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html Multi-colour PL-relations of Cepheids in the bt HIPPARCOS catalogue and the distance to the LMCWe analyse a sample of 236 Cepheids from the hipparcos catalog, usingthe method of reduced parallaxes'' in V, I, K and the reddening-freeWesenheit-index''. We compare our sample to those considered by Feast& Catchpole (1997) and Lanoix et al. (1999), and argue that oursample is the most carefully selected one with respect to completeness,the flagging of overtone pulsators, and the removal of Cepheids that mayinfluence the analyses for various reasons (double-mode Cepheids,unreliable hipparcos solutions, possible contaminated photometry due tobinary companions). From numerical simulations, and confirmed by theobserved parallax distribution, we derive a (vertical) scale height ofCepheids of 70 pc, as expected for a population of 3-10 Msunstars. This has consequences for Malmquist- and Lutz-Kelker (Lutz &Kelker 1973, Oudmaijer et al. 1998) type corrections which are smallerfor a disk population than for a spherical population. The V and I datasuggest that the slope of the Galactic PL-relations may be shallowerthan that observed for LMC Cepheids, either for the whole period range,or that there is a break at short periods (near log P_0 ~ 0.7-0.8). Westress the importance of two systematic effects which influence thedistance to the LMC: the slopes of the Galactic PL-relations andmetallicity corrections. In order to assess the influence of thesevarious effects, we present 27 distance moduli (DM) to the LMC. Theseare based on three different colours (V,I,K), three different slopes(the slope observed for Cepheids in the LMC, a shallower slope predictedfrom one set of theoretical models, and a steeper slope as derived forGalactic Cepheids from the surface-brightness technique), and threedifferent metallicity corrections (no correction as predicted by one setof theoretical models, one implying larger DM as predicted by anotherset of theoretical models, and one implying shorter DM based onempirical evidence). We derive DM between 18.45 +/- 0.18 and 18.86 +/-0.12. The DM based on K are shorter than those based on V and I andrange from 18.45 +/- 0.18 to 18.62 +/- 0.19, but the DM in K could besystematically too low by about 0.1 magnitude because of a bias due tothe fact that NIR photometry is available only for a limited number ofstars. From the Wesenheit-index we derive a DM of 18.60 +/- 0.11,assuming the observed slope of LMC Cepheids and no metallicitycorrection, for want of more information. The DM to the LMC based on theparallax data can be summarised as follows. Based on the PL-relation inV and I, and the Wesenheit-index, the DM is 18.60 ± 0.11(± 0.08 slope)(^{+0.08}_{-0.15} ;metallicity), which is ourcurrent best estimate. Based on the PL-relation in K the DM is ;;;;18.52 +/- 0.18 (± 0.03 ;slope) (± 0.06 ;metallicity)(^{+0.10}_{-0} ;sampling ;bias). The random error is mostly due to thegiven accuracy of the hipparcos parallaxes and the number of Cepheids inthe respective samples. The terms between parentheses indicate thepossible systematic uncertainties due to the slope of the GalacticPL-relations, the metallicity corrections, and in the K-band, due to thelimited number of stars. Recent work by Sandage et al. (1999) indicatesthat the effect of metallicity towards shorter distances may be smallerin V and I than indicated here. From this, we point out the importanceof obtaining NIR photometry for more (closeby) Cepheids, as for themoment NIR photometry is only available for 27% of the total sample.This would eliminate the possible bias due to the limited number ofstars, and would reduce the random error estimate from 0.18 to about0.10 mag. Furthermore, the sensitivity of the DM to reddening,metallicity correction and slope are smallest in the K-band. Based ondata from the ESA HP astrometry satellite. Direct calibration of the Cepheid period-luminosity relationAfter the first release of Hipparcos data, Feast & Catchpole gave anew value for the zero-point of the visual Cepheid period-luminosityrelation, based on trigonometric parallaxes. Because of the largeuncertainties on these parallaxes, the way in which individualmeasurements are weighted is of crucial importance. We thereforeconclude that the choice of the best weighting system can be aided by aMonte Carlo simulation. On the basis of such a simulation, it is shownthat (i) a cut-off in π or in σ_ππ introduces a strongbias; (ii) the zero-point is more stable when only the brightestCepheids are used; and (iii) the Feast & Catchpole weighting givesthe best zero-point and the lowest dispersion. After correction, theadopted visual period-luminosity relation is=-2.77logP-1.44+/-0.05. Moreover, we extend this study to thephotometric I band (Cousins) and obtain=-3.05logP-1.81+/-0.09. I- and JHK-band photometry of classical Cepheids in the HIPPARCOS catalogBy correlating the \cite[Fernie et al. (1995)]{F95} electronic databaseon Cepheids with the resolved variable catalog'' of the hipparcosmission and the simbad catalog one finds that there are 280 Cepheids inthe hipparcos catalog. By removing W Vir stars (Type ii Cepheids),double-mode Cepheids, Cepheids with an unreliable solution in thehipparcos catalog, and stars without photometry, it turns out that thereare 248 classical Cepheids left, of which 32 are classified asfirst-overtone pulsators. For these stars the literature was searchedfor I-band and near-infrared data. Intensity-mean I-band photometry onthe Cousins system is derived for 189 stars, and intensity-mean JHK dataon the Carter system is presented for 69 stars. Structural properties of s-Cepheid velocity curves Constraining the location of the omega_4 = 2omega_1 resonanceThe light curves of the first overtone Pop. I Cepheids (s-Cepheids) showa discontinuity in their phi_ {21} vs. {P} diagram, near {P} = 3.2 day.This feature, commonly attributed to the 2:1 resonance between the firstand the fourth overtones (omega_4 ~ 2omega_1 ), is not reproduced by thehydrodynamical models. With the goal of reexamining the resonancehypothesis, we have obtained new CORAVEL radial velocity curves for 14overtone Cepheids. Together with 10 objects of Krzyt et al.( te{krzyt}), the combined sample covers the whole range of overtoneCepheid periods. The velocity Fourier parameters display a strongcharacteristic resonant behavior. In striking contrast to photometricones, they vary smoothly with the pulsation period and show no jump at3.2 day. The existing radiative hydrodynamical models match the velocityparameters very well. The center of the omega_4 = 2omega_1 resonance isestimated to occur at {P}r = 4.58\pm 0.04 day, i.e. at aperiod considerably longer than previously assumed (3.2 day). Weidentify two new members of the s-Cepheid group: MYPup and V440 Per. Based on observationscollected at the European Southern Observatory (La Silla, Chile) and atthe Observatoire de Haute-Provence (France)} The shape and scale of Galactic rotation from Cepheid kinematicsA catalog of Cepheid variables is used to probe the kinematics of theGalactic disk. Radial velocities are measured for eight distant Cepheidstoward l = 300 deg; these new Cepheids provide a particularly goodconstraint on the distance to the Galactic center, R0. We model the diskwith both an axisymmetric rotation curve and one with a weak ellipticalcomponent, and find evidence for an ellipticity of 0.043 +/- 0.016 nearthe sun. Using these models, we derive R0 = 7.66 +/- 0.32 kpc andv(circ) = 237 +/- 12 km/s. The distance to the Galactic center agreeswell with recent determinations from the distribution of RR Lyraevariables and disfavors most models with large ellipticities at thesolar orbit. A search for evolutionary changes in the periods of low-amplitude Cepheids.Not Available Galactic kinematics of Cepheids from HIPPARCOS proper motionsThe Hipparcos proper motions of 220 Galactic Cepheids, together withrelevant ground-based photometry, have been analyzed. The effects ofGalactic rotation are very clearly seen. Mean values of the Oortconstants, A = 14.82 +/- 0.84 km/s kpc, and B = -12.37 +/- 0.64 km/skpc, and of the angular velocity of circular rotation at the sun, 27.19+/- 0.87 km/s kpc, are derived. A comparison of the value of A withvalues derived from recent radial velocity solutions confirms, withinthe errors, the zero-points of the period-luminosity andperiod-luminosity-color relations derived directly from the Hipparcostrigonometrical parallaxes of the same stars. The proper motion resultssuggest that the Galactic rotation curve is declining slowly at thesolar distance from the Galactic Center (-2.4 +/- 1.2 km/s kpc). Thecomponent of the solar motion towards the North Galactic Pole is foundto be +7.61 +/- 0.64 km/s. Based on the increased distance scale deducedin the present paper, the distance to the Galactic Center derived in aprevious radial velocity study is increased to 8.5 +/- 0.5 kpc. The Cepheid period-luminosity zero-point from HIPPARCOS trigonometrical parallaxesHipparcos trigonometrical parallaxes of Cepheid variables are used toderive a zero-point for the period-luminosity (PL) relation. Adopting aslope from the Large Magellanic Cloud (LMC), the relation is found to be=-2.81 log P-1.43. The standard error of the zero-point is0.10 mag. Together with metallicity corrections this corresponds to adistance modulus of 18.70+/-0.10 for the LMC and 24.77+/-0.11 for M31.Some implications of these results are discussed. Estimates of theHubble constant (H_0) that are based on Cepheid observations togetherwith an adopted LMC distance modulus of 18.50 will on average now needto be decreased by ~10 per cent. However, metallicity corrections, whichhave frequently been ignored, will result in the actual percentagechange varying with the sample of galaxies studied. Calibration of RRLyrae absolute magnitudes using the LMC and M31 Cepheid distancesimplies an age for the oldest Galactic globular clusters of ~11 Gyr. Theparallax data show that the period of Polaris corresponds to firstovertone pulsation. The White-Dwarf Companions of 56 Persei and HR 3643We have obtained low-dispersion IUE spectra of the stars 56 Persei (F4V) and HR 3643 (F7 II), as part of a survey of late-type stars with a1565A flux excess in the TD-1 ultraviolet sky survey. The IUE spectrumof each star reveals the presence of a hot white dwarf companion. We fitthe Ly-alpha profile and ultraviolet continuum using pure hydrogenmodels, but the distance of the primary star is also needed to uniquelyconstrain the white dwarf parameters. We derive Teff = 16,420 +- 420 K,log g= 8.46 +- 0.2 for the white dwarf companion to 56 Per, using thephotometric distance of 30.1 +- 2.8 pc. The implied white dwarf mass is0.90 +- 0.12 solar mass, considerably above the median mass (~0.6 solarmass) of single white dwarfs. The parameters of the white dwarf in HR3643 are not well constrained, mainly due to a large uncertainty in thedistance. By assuming a reasonable range of gravity for the white dwarf(7.3 < log g < 9.0), we derive -1.4 < M_V < 0.6 for the F7II star, adn 28,970 < Teff < 35,990 K for the white dwarf.Prompted by our detection of a white dwarf companion of a luminous Fstar, we have examined the IUE archives to assess the upper limits onpossible white dwarf companions to Cepheids. The detection of aCepheid--white dwarf binary would provide important insights concerningthe most massive progenitors of white dwarfs. Only for the cases ofalpha UMi and Beta Dor are existing IUE spectra of Cepheids sufficientlydeep to rule out the presence of a white dwarf companion. (SECTION:Stars) Photometric Parameters for Short-Period CepheidsAbstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....111.1313E&db_key=AST Colour excesses of F-G supergiants and Cepheids from Geneva photometry.A reddening scale for F-G supergiants and Cepheids is presented.Supergiants with low reddenings or in clusters form the basis of thecalibration. In this sense, it is entirely empirical. The data have beenobtained in the Geneva photometric system. Comparisons with otherreddening scales show no disagreement. The only problem is with Fernie'sscale for Cepheids (1990), where a systematic trend exists. Its originis not clear. It is suggested to extend the number of supergiants withindependently obtained colour excesses in order to test the existence ofa possible luminosity dependence of the calibration. A period-colourrelation for Cepheids is deduced, on the basis of the present reddeningcorrections. It gives strong support for V473 Lyr being a secondovertone pulsator. Photoelectric Observations of Southern Cepheids in 1995Not Available Study of period changes of small-amplitude cepheids in Perseus, Puppis, Scorpius, Scutum, Taurus, Triangulum Australe, Ursa Minor and Vela.Not Available New method to recognize s-CepheidsA new method of delineating sinusoidal or s-Cepheids is presented. Themethos uses the values of (V) (the mean intensity), V - Bar (the averagemagnitude), and Vmean (the value of the mean magnitude).Fourier coefficient data from galactic Cepheids is used to derive theseterms in the V band and the differences between the various terms showsystematic trends with increasing period. The Cepheids can be easilygrouped into 3 divisions-short period s-Cepheids, intermediate periodCepheids (P less than 9 days), and long period Cepheids (P greater than9 days). Cepheids previously designated as s-Cepheids by others arecompared to those found using the method outlined here. The method isalso applied to Cepheids in the Small Magellanic Cloud to examine itssuitability as a pulsation mode discriminator. Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with. A comparison between observations and nonlinear models of first overtone mode Cepheids.Light and radial velocity curves of observed first overtone modeCepheids have been compared with the curves of nonlinear modelsconstructed using the new opacities. The study confirms that thecharacteristics of the curves depend on the resonance P_1_/P_4_=2between the first and the fourth overtone mode. However, discrepanciesand systematic differences between theory and observations are stillpresent. It is shown that the diagrams of amplitude ratios R_21_ andphase difference φ_21_ of light curves versus the period aresensitive to the T_e_ of the models. Interesting properties of the phaselag between light and radial velocity curve are also discussed. New radial velocities for classical cepheids. Local galactic rotation revisitedNew centre-of-mass radial velocities are calculated for 107 classicalcepheids from CORAVEL observations. We generally determine thesevelocities from four to six measurements carefully spaced in phase, byfitting a "typical" radial velocity curve or the mirror image of thelight curve. A decomposition in Fourier series is used for stars withmore than 10 measurements. Distances are then computed through aperiod-luminosity-colour relation for 278 classical cepheids with knownradial velocity, and an axisymmetric galactic rotation model is appliedto the sample, using a generalised non-linear least square method withuncertainties on both the velocities and the distances. The bestresults, with a rotation curve modelled as a third order polynomial,are: Rsun_=8.09 +/-0.30 kpc, A=15.92 +/-0.34 km/s/kpc, 2ARsun_=257 +/-7 km/s, A2=d^2theta(R)/d R^2^=-3.38+/-0.38 km/s/kpc^2^, A3=d^3theta(R)/d R^3^=1.99 +/-0.62km/s/kpc^3^, u_0_=9.32 +/-0.80 km/s, v_0_=11.18 +/-0.65 km/s. The effectof modifying the distance scale of cepheids, the absorption coefficientor the fitting procedure algorithm are examined. It appears that theproduct 2 A Rsun_ is very robust towards these changes. Theextended sample of classical cepheids with known radial velocitypresented in this paper seems to imply a higher value for A thananterior studies. The radial velocity residuals show a systematic k-termof about 2 km/s. New evidence from cluster cepheids excludes anintrinsic cause for this shift, and a dynamical cause is proposed from acomparison with a N-body simulation of the Galaxy. The simulation showsthat a systematic bias of this magnitude is typical. The structure ofthe local residual velocity field is examined in some detail. UBV(RI)c observations for 13 bright CepheidsPhotometry in the UBV(RI)c system has been obtained for 13 of thebrightest Cepheids in the southern sky available to the new SydneyUniversity Stellar Interferometer. The stars observed were Eta Aq1, 1Car, U Car, AX Cir, Beta Dor, Zeta Gem, T Mon, Kappa Pav, U Sgr, W Sgr,X Sgr, Y Sgr, and AH Vel. The main purpose of the photometry is toprovide current epoch data for the phases of the variables, although forseveral of the stars these data are also the first in the (RI)c system. A magnitude-limited survey of Cepheid companions in the ultravioletResults of a magnitude-limited survey of classic Cepheids brighter than8th mag carried out to search for hot main-sequence companions arepresented. Spectra of 76 stars obtained with the IUE satellite in the2000-3200-A region were compared with the spectra of nonvariablesupergiants and also the single Cepheid Delta Cep to search for excessflux at 2500 A from possible companions. Photometric companions werefound for 21 percent of the sample. When the Cepheids known to be binaryfrom either orbital motion or spectra in the 1200-2000-A region areincluded, the percentage of companions rises to 29 percent. If astatistical correction from stars with orbital motion is included, 34percent have companions. This percentage is compared with that found byAbt et al. (1990) for B2-B5 main-sequence stars. If only systems withperiods longer than a year and separations not more than 30 arcsec areconsidered, only 18 percent of the B stars will become Cepheids withcompanions.
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