Predicted and Empirical Radii of RR Lyrae Stars

We present new theoretical Period-Radius-Metallicity (PRZ) relations for RRLyrae stars. Current predictions are based on a large set of nonlinear,convective models that cover a broad range of chemical abundances and inputparameters. We also provide new and homogeneous estimates of angular diametersfor a sample of field RR Lyrae stars using a recent calibration of theBarnes-Evans surface brightness relation. Predicted and empirical radii are,within the errors, in reasonable agreement, but in the short-period range thelatter present a larger scatter. As a working hypothesis we suggest that thisdiscrepancy might be due to the occurrence either of nonlinear features such asbumps or a steep rising branch. New distance determination for RR Lyr itself isin very good agreement with HST trigonometric parallax and with pulsationparallax.Comment: 12 pages, 3 figures, accepted on ApJ Letter