1994PASP..106..472M

We present Stromgren photometry of the two Population II variables XX Vir and V716 Oph. Our analysis of the photometry yields the following physical parameters of the variables: for XX Vir <Teff> = 6410 K, [Fe/H] = 2.0, and <logg> = 1.95; for V716 Oph <Teff> = 6450 K, [Fe/H]=2.1, and <log g> = 1.67. The <log g> values of these two metal-poor variables are considerably smaller than the metal-strong variable BL Herculis (<log g≥2.37). The big surprise in this investigation is that the surface gravity of V716 Oph (P=1.12), the shorter-period variable, is smaller than the surface gravity of XX Vir (P=1.35), the longer period variable. With the aid of P square root of rho = Q relation we derive a semi-theoretical P-L relation for the Type II Cepheids of the form M_V=-1.88 log P+0.13. This expression is in rough agreement (within the uncertainties of many quantities used in deriving the expression) with the observational data. The relatively gentle slope of the P-L relation (compared with the slope of the P-L relations of other pulsating stars) is probably a consequence of the fact that the masses of the Type II Cepheids are constant or nearly constant, independent of period, and a large variations of the M/R values with period.