  We derive the basic physical parameters of the field double-mode RR Lyrae
star BS Com from its observed periods and the requirement of consistency
between the pulsational and evolutionary constraints. By using the current
solar-scaled horizontal branch evolutionary models of Pietrinferni et al.
(2004) and our linear non-adiabatic purely radiative pulsational models, we get
M/M(Sun) = 0.698 +/- 0.004, log(L/L(Sun)) = 1.712 +/- 0.005, T(eff) = 6840 +/-
14 K, [Fe/H] = -1.67 +/- 0.01, where the errors are standard deviations
assuming uniform age distribution along the full range of uncertainty in age.
The last two parameters are in a good agreement with the ones derived from the
observed BVIc colours and the updated ATLAS9 stellar atmosphere models. We get
T(eff) = 6842 +/- 10 K, [Fe/H] = -1.58 +/- 0.11, where the errors are purely
statistical ones. It is remarkable that the derived parameters are nearly
independent of stellar age at early evolutionary stages. Later stages,
corresponding to the evolution toward the asymptotic giant branch are most
probably excluded because the required high temperatures are less likely to
satisfy the constraints posed by the colours. We also show that our conclusions
are only weakly sensitive to nonlinear period shifts predicted by current
hydrodynamical models.
