  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}.
