Variable Stars in the Ursa Minor Dwarf Galaxy

The system of ~95 variable stars in the Ursa Minor (UMi) dwarf galaxy has been studied using two new sets of photometry (derived from 22 photographic plates and 38 CCD frames taken in 1984), combined with van Agt's (1967, 1968) photometry of Baade's photographic plates. Accurate pulsation periods are derived for 88 variable stars, three of which were discovered near the outer edge of the galaxy. Despite the existence of many relatively long-period c type RR Lyrae stars, an extensive search revealed no double-mode RR Lyrae stars. Among the shortest-period c type RR Lyrae stars, one possible second-overtone pulsator was identified. The mean period of the 47 ab type RR Lyrae stars is 0.638^d^, the mean period of the 35 c type RR Lyrae stars is 0.375^d^, and the fraction of c type RR Lyrae stars is 43%, all of which are characteristic of an Oosterhoff Type II system with a low mean metal abundance. Period-amplitude and period-rise-time diagrams are plotted and discussed. A c-m diagram for UMi, which is based on CCD photometry and includes the RR Lyrae stars, shows that the blue and red edges of the instability strip occur at B-V=0.16 + 0.03 mag and 0.40 + 0.05 mag, respectively. The ab type and c type RR Lyrae stars probably have overlapping color distributions, as is seen in the Galactic globular clusters M15 and ω Cen. The distance modulus for UMi is (m - M)_0_ = 19.24+/-0.24 mag, corresponding to a distance of 70 +/- 9 kpc. In addition to the three previously known anomalous Cepheids in UMi (V6, V56, and V59), four new anomalous Cepheids (V1, V11, V62, and V80) have been identified. The ratio of blue stragglers to anomalous Cepheids in UMi is shown to be ~10, which is consistent with the ratio predicted by the 'mass transfer in a binary system' hypothesis. In a period-luminosity diagram for the anomalous Cepheids in UMi, Draco, Sculptor, Leo II, and NGC 5466, two distinct linear relationships are seen, one corresponding to fundamental-mode pulsators, and the other corresponding to first-overtone pulsators. Five of the UMi anomalous Cepheids are fundamental-mode pulsators, and two are first-overtone pulsators. By using the P-L diagram to establish the true pulsation modes for anomalous Cepheids, much more reliable mass estimates can now be made. The minimum, maximum, and mean mass of the five anomalous Cepheids in Draco are shown to be 1.04, 1.77, and 1.38 M_sun_, respectively. Anomalous Cepheids in other galaxies are also discussed.