Cepheid Metallicity in the Leavitt Law (C- MetaLL) survey

Context. Classical Cepheids (DCEPs) play a fundamental role in the calibration of the extragalactic distance ladder, which eventually leads to the determination of the Hubble constant ( H 0 ) thanks to the period–luminosity (PL) and period–Wesenheit (PW) relations exhibited by these pulsating variables. Therefore, it is of great importance to establish the dependence of PL and PW relations on metallicity. Aims. We aim to quantify the metallicity dependence of the PL and PW relations of the Galactic DCEPs for a variety of photometric bands, ranging from optical to near-infrared. Methods. We gathered a literature sample of 910 DCEPs with available Fe/H values from high-resolution spectroscopy or metallicities from the Gaia Radial Velocity Spectrometer. For all these stars, we collected photometry in the G BP , G RP , G , I , V , J , H , and K S bands and astrometry from Gaia Data Release 3 (DR3). We used these data to investigate the metal dependence of both the intercepts and slopes of a variety of PL and PW relations at multiple wavelengths. Results. We find a large negative metallicity effect on the intercept ( γ coefficient) of all the PL and PW relations investigated in this work, while present data still do not allow us to draw firm conclusions regarding the metal dependence of the slope ( δ coefficient). The typical values of γ are around −0.4 : −0.5 mag dex −1 , which is larger than most of the recent determinations present in the literature. We carried out several tests, which confirm the robustness of our results. As in our previous works, we find that the inclusion of a global zero point offset of Gaia parallaxes provides smaller values of γ (in an absolute sense). However, the assumption of the geometric distance of the Large Magellanic Cloud seems to indicate that larger values of γ (in an absolute sense) would be preferred.