Masses of AC Andromedae, U Trianguli Australis, and BC Draconis using radiative models

Observed periods of AC And (PI0 = 0.711, PI1 = 0.525, PI2 = 0.421 day), U Tr A (PI0 = 2.568, PI1 = 1.825 days), and BC Dra (PI1 = 3.351, PI2 = 2.566 days) are used together with results from the radial linear nonadiabatic pulsation theory to obtain their masses, radii, luminosities and compositions. AC And is unique in having three identifiable periods, and even though it is classified as an RR Lyrae variable, its mass must be larger than normal for these stars, that is, (1.4 +- 0.3) M/sub solar mass/. Its radius is (8.2 +- 0.7) R/sub solar mass/, and its luminosity (100 +- 40) L/sub solar mass/ in order to be in the pulsation instability strip. U Tr A is typical of the double mode or beat Cepheids with a mass of (1.2 +- 0.2) M/sub solar mass/ its radius (17 +- 1) R/sub solar mass/ and its luminosity (315 +-90)L/sub solar mass/. BC Dra cannot be explained by assigning the two periods to PI0 and PI1, but if its pulsation is in the first and second overtones, its mass is (3.6 +- 0.4) M/sub solar mass/, its radius (37 +- 2) R/sub solar mass/ and its luminosity (1200 +- 100) L/sub solar mass/. All these variables seem to be explained by normal population I compositions. These masses are all low compared to evolutionary theory which does not even predict evolution to the instability strip following helium ignition in the core at a mass below 3 M/sub solar mass/. The double mode mass, M/sub D/, of BC Dra is 0.74 that given by evolution theory at a luminosity needed to be in the instability strip