  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.
