Fundamental parameters of Cepheids IV . Radii and luminosities

A temperature scale for Cepheids is presented, based on Geneva photometry. It uses new atmosphere models computed at various values of microturbulent velocity. The scale so-obtained is compared to other ones and the importance of microturbulence effects is shown. This calibration is applied to 20 Cepheids for which the variation of microturbulence is known. The detailed variations of temperature and gravity are derived. The behavior of the pho-tometric gravity is in very good agreement with the effective gravity (sum of GM/R 2 and of the derivative of the radial velocity). Thus static atmosphere models can be used to describe the temperature and gravity variations in Cepheids under the following conditions: i) the microturbulence has to be taken into account, ii) the effective gravity must be considered instead of the static gravity, iii) in some cases, a small phase interval around minimum radius does not give reliable results. The temperature and bolometric corrections are then used to derive radii and distances via the Baade-Wesselink technique. The resulting Period-Radius and Period-Luminosity relations are discussed. Our results compare very well with similar analysis based on infra-red (JHK) photometry.