  Our photometric-hydrodynamic method is generalized to determine fundamental
parameters of multiperiodic radially pulsating stars. We report 302 UBV(RI)_C
Johnson-Kron-Cousins observations of GSC 4868-0831. Using these and published
photometric data of V372 Ser, their metallicity, reddening, distance, mass,
radius, equilibrium luminosity, effective temperature are determined. The
results underline the necessity of using multicolour photometry including an
ultraviolet band to classify properly subgroups of RR Lyrae stars: our U
observations could reveal that GSC 4868-0831 is a subgiant star pulsating in
two radial modes, V372 Ser is a giant star with size and mass of an RRd star.
