FUNDAMENTAL PARAMETERS OF CEPHEIDS .4. RADII AND LUMINOSITIES

A temperature scale for Cepheids is presented, based on Geneva photome try. It uses new atmosphere models computed at various values of micro turbulent velocity. The scale so-obtained is compared to other ones an d 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 deriv ed. The behavior of the photometric 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 descri be the temperature and gravity variations in Cepheids under the follow ing conditions: i) the microturbulence has to be taken into account, i i) the effective gravity must be considered instead of the static grav ity, iii) in some cases, a small phase interval around minimum radius does not give reliable results. The temperature and bolometric correct ions are then used to derive radii and distances via the Baade-Wesseli nk technique. The resulting Period-Radius and Period-Luminosity relati ons are discussed. Our results compare very well with similar analysis based on infra-red (JHK) photometry.