2008A&A...488...25G

The metallicity dependence of the Cepheid PL-relation is of importance in establishing the extra-galactic distance scale. The aim of this paper is to investigate the metallicity dependence of the PL-relation in V and K based on a sample of 68 Galactic Cepheids with individual Baade-Wesselink distances (some of the stars also have an HST-based parallax) and individually determined metallicities from high-resolution spectroscopy. Literature values of the V-band, K-band and radial velocity data have been collected for a sample of 68 classical cepheids that have their metallicity determined in the literature from high-resolution spectroscopy. Based on a (V-K) surface-brightness relation and a projection factor derived in a previous paper, distances have been derived from a Baade-Wesselink analysis. PL- and PLZ-relations in V and K are derived. The effect of the adopted dependence of the projection factor on period is investigated. The change from a constant p-factor to one recently suggested in the literature with a mild dependence on logP results in a less steep slope by 0.1 unit, which is about the 1-sigma error bar in the slope itself. The observed slope in the PL-relation in V in the LMC agrees with both hypotheses. In K the difference between the Galactic and LMC slope is larger and would favour a mild period dependence of the p-factor. The dependence on metallicity in V and K is found to be marginal, and independent of the choice of p-factor on period. This result is severely limited by the small range in metallicity covered by the Galactic Cepheids.