EROS differential studies of Cepheids in the Magellanic Clouds : Stellar pulsation, stellar evolution and distance scale

We present a differential study of 500 Magellanic Cepheids with 3 million measurements obtained as a by-product of the EROS microlensing survey. The data-set is unbiased and provides an excellent basis for a differential analysis between LMC and SMC. We investigate the pulsational properties of the Cepheids as a function of metallicity and the constraints they give to stellar pulsation, stellar evolution theory and opacities. Moreover, our empirical analysis shows a small metallicity effect on the period-luminosity relation which has a significant implication to recent distance estimates based on HST observations of Cepheids in galaxies as far as the Virgo cluster. When the color shift of Cepheids due to metallicity is interpreted as reddening in the determination of the true distance modulus of a target galaxy a systematic error is made. Accounting for that error makes the low-H_0 values (Sandage et al.) higher and the high-H_0 (Freedman et al.) value lower, bringing those discrepant estimates into agrement around $H_0 \approx 70$ km/s Mpc^{-1}.