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Physical properties of double-mode RR Lyrae stars based on pulsation and evolution models

We determine the fundamental stellar parameters of altogether 20 double-mode RR Lyrae (RRd) stars from the Galactic Field and the Large Magellanic Cloud, following our approach employed on the field RRd star BS Comae. The stars were selected to cover wide ranges of periods and period ratios, implying diverse stellar parameters. From the possible observed quantities we use only the periods and determine stellar parameter combinations that satisfy both current helium-burning evolutionary models and grids of linear non-adiabatic purely radiative pulsational models. Thus, the periods of an object determine a sequence of solutions for its mass, luminosity, effective temperature and metallicity, parametrized by the time elapsed from the zero age horizontal branch. The derived sets of solutions yield various important theoretical relations between the physical parameters of the stars which are, in the case of some parameter combinations, nearly independent of the age. We get very tight simple linear relations between log(P0) and log(R), log(rho), log(g) and the Wesenheit index W(B-V). This latter period-luminosity-color relation is in good agreement with the one derived on empirical basis and calibrated by Baade-Wesselink results.