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We determine the fundamental stellar parameters of altogether 20 double-modeRR 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 wereselected to cover wide ranges of periods and period ratios, implying diversestellar parameters. From the possible observed quantities we use only theperiods and determine stellar parameter combinations that satisfy both currenthelium-burning evolutionary models and grids of linear non-adiabatic purelyradiative pulsational models. Thus, the periods of an object determine asequence of solutions for its mass, luminosity, effective temperature andmetallicity, parametrized by the time elapsed from the zero age horizontalbranch. The derived sets of solutions yield various important theoreticalrelations between the physical parameters of the stars which are, in the caseof some parameter combinations, nearly independent of the age. We get verytight simple linear relations between log(P0) and log(R), log(rho), log(g) andthe Wesenheit index W(B-V). This latter period-luminosity-color relation is ingood agreement with the one derived on empirical basis and calibrated byBaade-Wesselink results.

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