An investigation of the revised CORS method based on theoretical models

Direct measurements of Cepheid radii are a key for understanding the physic al structure of these variables, and in turn for constraining their pulsati on properties. In this paper we discuss the numerical experiments we perfor med for testing the accuracy of Cepheid radii obtained by adopting both the pure Baade-Wesselink (BW) method and the revised CORS method, as well as t he consistency of the physical assumptions on which these methods are based . We applied both the BW and the revised CORS methods to the synthetic ligh t, color and radial velocity curves predicted by Cepheid full amplitude, no nlinear, convective models at solar chemical composition. We found that these methods systematically either underestimate or overesti mate "theoretical" radii if radius determinations are based on optical (BVR ) bands or on (VJK) bands, respectively. At the same time, current simulati ons suggest that CORS radii are in very good agreement with "theoretical" r adii if the surface brightness is calibrated by adopting a bidimensional fi t of atmosphere models which accounts for temperature, gravity, and bolomet ric correction variations along the pulsation cycle. Finally, a slight discrepancy between "computed" and "theoretical" radii of a Bump Cepheid supports the exclusion of these pulsation phases in both BW and CORS analyses. In fact, we found that the assumption of quasi-static a pproximation is no longer valid during the pulsation phases in which appear s the bump.