Cepheids: the period-luminosity relation

The Cepheids are relatively young, bright, periodic supergiant variable stars showing a correlation between their periods and luminosities. Since the beginning of the century, the Cepheid Period-Luminosity relation has been the corner stone of distance determination, and of the measure of the correlation between the apparent relative expansion velocity to the relative distance (the Hubble constant, H-0). Using data obtained from EROS project, we empirically found a small effect of metallicity on the Period-Luminosity relation which has important consequences for the inferred Cepheid distances and the determination of H-0.