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The Cepheid period-luminosity (PL) relation is unquestionably one of the mostpowerful tools at our disposal for determining the extragalactic distancescale. While significant progress has been made in the past few years towardsits understanding and characterization both on the observational andtheoretical sides, the debate on the influence that chemical composition mayhave on the PL relation is still unsettled. With the aim to assess theinfluence of the stellar iron content on the PL relation in the V and K bands,we have related the V-band and the K-band residuals from the standard PLrelations of Freedman et al. (2001) and Persson et al. (2004), respectively, to[Fe/H]. We used direct measurements of the iron abundances of 68 Galactic andMagellanic Cepheids from FEROS and UVES high-resolution and highsignal-to-noise spectra. We find a mean iron abundance ([Fe/H]) about solar(sigma = 0.10) for our Galactic sample (32 stars), -0.33 dex (sigma = 0.13) forthe Large Magellanic Cloud (LMC) sample (22 stars) and -0.75 dex (sigma = 0.08)for the Small Magellanic Cloud (SMC) sample (14 stars). Our abundancemeasurements of the Magellanic Cepheids double the number of stars studied upto now at high resolution. The metallicity affects the V-band Cepheid PLrelation and metal-rich Cepheids appear to be systematically fainter thanmetal-poor ones. These findings depend neither on the adopted distance scalefor Galactic Cepheids nor on the adopted LMC distance modulus. Current data donot allow us to reach a firm conclusion concerning the metallicity dependenceof the K-band PL relation. The new Galactic distances indicate a small effect,whereas the old ones support a marginal effect.

The influence of chemical composition on the properties of Cepheid stars. II-The iron content