We report the most comprehensive UBV(RI)_C observations of the bright,
radially pulsating field stars SW And, DH Peg, CU Com, DY Peg. Long term
variation has been found in the ultraviolet colour curves of SW And and DH Peg.
We apply our photometric-hydrodynamic method to determine the fundamental
parameters of these stars: metallicity, reddening, distance, mass, radius,
equilibrium luminosity and effective temperature. Our method works well for SW
And, CU Com and DY Peg. A very small mass 0.26+/-0.04 M_Sun of SW And has been
found. The fundamental parameters of CU Com are those of a normal double-mode
RR Lyrae (RRd) star. DY Peg has been found to have paradoxical astrophysical
parameters: the metallicity, mass and period are characteristic for a
high-amplitude Delta Sct star while the luminosity and radius place it in the
group of RR Lyrae stars. DH Peg has been found to be peculiar: the definite
instability in the colour curves towards ultraviolet, the dynamical variability
of the atmosphere during the shocked phases suggests that the main assumptions
of our photometric-hydrodynamic method, the quasi-static atmosphere
approximation (QSAA) and the exclusive excitation of radial modes are probably
not satisfied in this star. The fundamental parameters of all stars studied in
this series of papers are summarized in tabular and graphical form.Comment: 15 pages, 10 figures; accepted for publication in MNRA
