I demonstrate that the two unexpected results in the local Universe: anomalous intrinsic (V – I)0 colors of RR Lyrae stars and clump giants in the Galactic center, and very short distances to Magellanic Clouds inferred from clump giants, can be at least partially resolved with a modified coefficient of selective extinction AV/E(V – I). With this modification, I find a new clump-giant distance modulus to the Large Magellanic Cloud, μLMC = 18.27 ± 0.07, which is 0.09 larger than the Udalski (1998b) result. When distance estimates from the red clump, RR Lyrae stars and the eclipsing binary HV2274 are combined, one obtains μLMC = 18.31 ± 0.04 (internal).

The Optical Gravitational Lensing Experiment (OGLE) data on high-amplitude δ Scuti Stars (HADS) and RR Lyrae stars have been analyzed to determine the distance to the galactic bulge. We find a mean value of 14.47 ± 0.08 mag for the bulge distance modulus. A small additive correction leads to 7.9 ± 0.3 kpc as the best distance to the galactic center.

The population of Cepheids in the Hipparcos catalogue is retrieved. Some observational properties of the sample are modelled using a. Monte Carlo code. This code is then used to verify that the bias introduced by the method of “reduced parallax” in deriving the zero point of the P–L relation is of order 0.01 mag, confirming earlier results. This bias is likely due to Malmquist bias.

Cepheids are the pillar of the extragalactic distance scale, but their reach in distance is not sufficient to calibrate H0 Yet HST has provided Cepheid distances to eight galaxies which have produced SNe la. The latter are used as nearly perfect standard candles to carry the distance scale to 30 000 km s−1, giving H0 = 58 ± 5. This determination finds support in other methods, but its range in distance and its weight is unparalleled.

We have applied an approximately Bayesian and a fully Bayesian analysis to the calculation of Cepheid distances, radii and absolute magnitudes using the surface brightness (Baade–Wesselink) method. Both methods successfully account for errors in the data, provide unbiased distance estimates, and provide objective model selection for the radial velocity curve. In addition, the fully Bayesian analysis objectively selects a model for the magnitude curve; averages over models of various Fourier orders, properly weighted by the posterior probabilities of the individual models; and includes a Lutz–Kelker correction.

The approximately Bayesian method is that described by Jefferys & Barnes (1999) and Barnes & Jefferys (1999). It is a maximum likelihood approach with objective selection of the order of the Fourier series model of the radial velocities.

A reanalysis of the HST data on Cepheids in the Virgo cluster spiral M100 is presented, based on our study of the Galactic Cepheids. A distance modulus of 31.49 mag is derived, which is higher than the HST Key Project value of 31.04 mag obtained from the same data earlier (Ferrarese et al. 1996). The increase in the distance is caused by nearly equal contributions from a different calibration of the slope and the zero point of the period–luminosity relation and a correction for the flux-limited incompleteness bias.

A search for Cepheids in NGC 4414 has been performed using HST archive data. Accurate photometry for this galaxy is challenging due to its high surface brightness background with steep gradients and the rather low S/N of stellar point sources. The photometry package ROMAFOT has been used, and the results compare favorably with published photometry reported by Turner et al. (1998). All eleven Cepheids were recovered and nine additional candidates were discovered. The resulting distance modulus of 31ṃ50±0ṃ27 is only 0ṃ1 larger than previously reported.

The shapes of light curves and of radial velocity curves are two main predictions of the hydrodynamical models of Cepheids. Of the two, the velocity curves are more robust numerically and therefore, more suitable for comparison with the observations. In this report, we present accurate Fourier parameters for an extensive set of classical Cepheid velocity curves. Published radiative models reproduce the observations very well, with only small discrepancies present. We estimate the center of the ω2 = 2ω0 resonance to occur at Pr = 9.947 ± 0.051 day

It is a well-known property of Cepheids and RR Lyrae variables that their maximum brightness approximately coincides with the maximum expansion velocity. A more detailed investigation (Carson & Stothers 1984; Simon 1984) has shown that the velocity curve is in fact slightly retarded with the respect to the light variations. In this report we discuss the trends in the observed phase lag for an extensive sample of classical Cepheids and RRab variables.

Polaris (α UMi) is the only classical Cepheid with Hipparcos parallax determined to better than 10% (Feast & Catchpole 1997). The correct identification of its pulsation mode is therefore crucial for calibrating the Cepheid period–luminosity relation. We show that Polaris is an overtone pulsator. Our analysis confirms the finding of Feast & Catchpole, but is entirely independent of the distance estimation.

The light curves of the first overtone Pop I Cepheids (s-Cepheids) show a discontinuity in their ϕ21 vs. P diagram, near P = 3.2 d. This feature, commonly attributed to the 2:1 resonance between the first and the fourth overtones (ω4 ≈ 2ω1), is not reproduced by the hydrodynamical models (Antonello & Aikawa 1995). With the goal of reexamining the resonance hypothesis, we have obtained new CORAVEL and CORALIE radial velocity curves for 17 overtone Cepheids. Together with 10 objects from Krzyt et al. (2000), the combined sample covers the whole range of overtone Cepheid periods. The velocity Fourier parameters display a strong characteristic resonant behavior. In striking contrast to photometric ones, they vary smoothly with the pulsation period and show no jump at 3.2 d. The existing radiative hydrodynamical models match very well the velocity parameters. The center of the ω4 = 2ω1 resonance is estimated to occur at Pr = 4.58 ± 0.04 d, i.e. at a period considerably longer than previously assumed (3.2 d). Five new s-Cepheids, with periods above 5 d, have been identified from the radial velocity curves morphology: V419 Cen, V659 Cen, MY Pup, GH Car and V440 Per. The data and the discussion can be found in Kienzle et al. (1999). Preprints and the data are available at: http://obswww.unige.ch/˜kienzle.

Our systematic measurements (since 1987) of radial velocities of northern Cepheids with a correlation spectrometer allowed us to make a detailed study of shapes of VR curves. This is a preliminary analysis of the phase lag between Cepheid radial velocity and light curves.

A sequence similar to Hertzsprung progression was revealed for Cepheid radial-velocity curves. We separated two pulsation modes for six double-mode Cepheids and determined radii for five of them. Several new spectroscopic-binary Cepheids were discovered; we present new preliminary orbital periods for V496 Aql, VY Cyg, and V1334 Cyg, in a combined table of our results on Cepheid binarity.

Our analysis of O – C diagrams for 230 (mainly northern) Cepheids revealed that periods of 67 of them show evolutionary changes. New parabolic elements for these Cepheids are presented.

A preliminary orbit has been derived for the high luminosity, 27-d classical Cepheid T Mon. Velocities for the hot companion have been measured from an HST GHRS spectrum and 3 IUE spectra. The companion velocities are inconsistent with binary orbital motion and it is likely that the companion is itself a short period binary. The HST spectrum also shows that the companion is a chemically peculiar star, probably magnetic.

Two parameters are defined involving the observed (peak-to-peak) amplitudes in U, B, V, and R photometric bands as well as that of the radial velocity variation. They are used to detect companions to Cepheids for a sample about 300 Galactic Cepheids.

We have computed stellar evolutionary models for stars in a mass range characteristic of Cepheid variables for metallicities representative of the Magellanic Clouds populations. These calculations are coupled to a linear non-adiabatic stability analysis in order to get self-consistent mass-period-luminosity relationships. We construct period histograms taking into account a mass function and evolutionary time scales, and we compare them to those given by the microlensing surveys EROSl and OGLE.

Preliminary results are reported for a monitoring program on ω Cen. We search for multi-mode SX Phe stars and changes in pulsation parameters of the cluster variables in order to test models of stellar evolution. With a periodogram for 10,000 light curves, we estimate that ω Cen hosts several hundred SX Phe stars.

The periods of most of the RRab stars in ω Centauri have increased during the past century at the same rate as evolutionary model calculations predict. This result raises the possibility that evolutionary changes in the periods are directly observed in this cluster.

We present new multicolour CCD photometry of the central part of the globular cluster M3, mapping the precise position of ~ 120 RR Lyrae stars (RRab, RRd, RRc) on the horizontal branch (HB). The location of the double-mode variables (RRd) is in perfect agreement with recent theoretical results. We find a significant internal spread of metallicity amongst the RRab variables.