Galactic Cepheids with Gaia DR2 : Period-Luminosity relations and implications on H_0

Cepheids represent a fundamental tool for measuring the distances in the Universe, thanks to the simple correlation between their pulsation periods and their instrinsic luminosity: the period-luminosity (PL) relation. In order to calibrate this relation accurately, precise distance measurements are required. However, the recent data releases of the Gaia satellite show that Cepheids parallaxes are subject to biases due to saturation and to the large amplitude of their color variation, which makes the improvement of the PL calibration impossible. In order to bypass this bias, we use the parallaxes of Cepheids detached companions as a proxy for the Cepheids parallaxes themselves, since they are stable and classical stars, to calibrate the Leavitt law. This new method also allowed us to estimate a value of 69 ± 2 km/s/Mpc for the Hubble constant, in agreement with the determination from the Planck satellite.