Period Changes in SX Phoenicis Stars. II. XX Cygni

We have conducted photometric monitoring of the SX Phoenicis star XX Cyg in order to study the changes in pulsation period of this object. We have combined 20 new times of maximum light for XX Cyg with those from the literature to prepare an O-C diagram for XX Cyg. The results suggest that contrary to earlier work, XX Cyg exhibits a continuous period increase, rather than sudden jumps in the period. We measure the change in period of XX Cyg to be (1/P)(dP/dt)=1.13(2)×10^-8 yr^-1. This is in reasonably good agreement with the theoretical predictions of Rodríguez et al. of (1/P)(dP/dt)=6.2×10^-8 yr^-1 change expected from theoretical considerations. We marginally detect a bump in the descending portion of the light curve of XX Cyg, as reported by previous authors.