Very metal poor classical cepheids: the distance of IZw18

We have obtained deep multi-band (V, I) HST/ACS time-series photometry of the very metal-poor Blue Compact dwarf galaxy IZw18, with the aim of constraining the distance and the Star Formation History of this galaxy (see Aloisi et al. 2007). The timeseries sequences allowed us to identify about thirty candidate variables in the galaxy, and to obtain well sampled light curves and reliable periods for three Classical Cepheids, with periods of 8.63, 124 and 130 days, respectively, and a long period variable, with possible period of 139 days. These data provide new insight into the properties of the Classical Cepheids at very low metallicity regime. We have applied to the IZw18 Cepheids theoretical Wesenheit (V, I) relations based on new pulsation models of Classical Cepheids specifically computed for the extremely low metallicity of this primordial galaxy (Z=0.0004, Y=0.24, Marconi et al. 2007 in preparation). As a result we obtain an estimate for the IZw18 mean distance modulus of 31.35 ± 0.26 mag, with canonical models, and of 31.09 ± 0.26 mag, with overluminous models. These distances, and the former in particular, are in very good agreement with the distance we infer from the galaxy Red Giant Branch Tip.