The effect of metallicity on the Cepheid distance scale and its implications for the Hubble constant (H₀) determination

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Recent HST determinations of the expansion's rate of the Universe (the Hubble constant, H₀) assumed that the Cepheid Period-Luminosity relation at V and I are independent of metallicity (Freedman, et al., 1996, Saha et al., 1996, Tanvir et al., 1995). The three groups obtain different vales for H₀. We note that most of this discrepancy stems from the asumption (by both groups) that the Period-Luminosity relation is independent of metallicity. We come to this conclusion as a result of our study of the Period-Luminosity relation of 481 Cepheids with 3 millions two colour measurements in the Large Magellanic Cloud and the Small Magellanic Cloud obtained as a by-product of the EROS microlensing survey. We find that the derived interstellar absorption corrections are particularly sensitive to the metallicity and when our result is applied to recent estimates based on HST Cepheids observations it makes the low-H₀ values higher and the high-H₀ value lower, bringing those discrepant estimates into agrement around H₀ ≈ 70 km/s Mpc⁻¹.