METALLICITY EFFECTS ON THE CEPHEID EXTRAGALACTIC DISTANCE SCALE FROM EROS PHOTOMETRY IN THE LARGE-MAGELLANIC-CLOUD AND THE SMALL-MAGELLANIC-CLOUD

This is an investigation of the period-luminosity relation of classica l Cepheids in samples of different metallicity. It is based on 481 Cep heids in the Large and Small Magellanic Clouds from the blue and red f ilter CCD observations (most similar to V-J & R-J) of the French EROS microlensing project, The data-set is complete and provides an excelle nt basis for a differential analysis between LMC and SMC. In compariso n to previous studies of effects on the PL-relation, the EROS dataset offers extremely well-sampled light curves and well-filled instability strips. This allows reliable separation of Cepheids pulsating in the fundamental and the first overtone mode and derivation of differential reddening. Our main result concerns the determination of distances to galaxies which are inferred by using the LMC as a base and using two color photometry to establish the amount of reddening. We find a zero- point offset between SMC and LMC which amounts to a difference between inferred and true distance modulus of 0.14+/-0.06 mag in the VIc syst em, The offset is exactly the same in both sets of PL-relations - of t he fundamental and of the first overtone mode Cepheids. No effect is s een on the slopes of the PL-relations, although the fundamental and th e first overtone mode Cepheids have different PL slopes. We attribute the color and the zero-point offset to the difference in metallicity b etween the SMC and LMC Cepheids. A metallicity effect of that small ma gnitude still has important consequences for the inferred Cepheid dist ances and the determination of H-0. When applied to recent estimates b ased on HST Cepheid observations, our metallicity dependence makes the low H-0 values (Sandage et al. 1994) higher and the high-H-0 values ( Freedman et al, 1994b) lower, thus bringing those discrepant estimates into agreement near H-0 similar to 70 km.s(-1) Mpc(-1).