SX Phoenicis Stars in the Core of 47 Tucanae

We present new results on five of six known SX Phoenicis stars in the core of the globular cluster 47 Tucanae. We give interpretations of the light curves in the V and I bands from 8.3 days of observations with the Hubble Space Telescope near the core of 47 Tuc. The most evolved SX Phe star in the cluster is a double-mode pulsator (V2) and we determine its mass to be 1.54+/-0.05 solar masses from its position in the Hertzsprung-Russell diagram and by comparing observed periods with current theoretical pulsation models. For V14 we do not detect any pulsation signal. For the double-mode pulsators V3, V15, and V16 we cannot give a safe identification of the modes. We also describe the photometric techniques we have used to extract the light curves of stars in the crowded core. Some of the SX Phoenicis are saturated and we demonstrate that even for stars that show signs of a bleeding signal we can obtain a point-to-point accuracy of 1-3%.