Pulsations and Period Changes of the SX Phoenicis Star DY PegasiBased on data obtained at Xinglong Station of the National Astronomical Observatories, and Lijiang Station of Yunnan Astronomical Observatory, Chinese Academy of Sciences, China.

We present 83 new times of maximum light of the SX Phoenicis (SX Phe) star DY Pegasi (DY Peg), based mainly on our new time-series photometric observations from the years 2004-2008. Together with the times of maximum light in the literature, a comprehensive study of the O- Cdiagram with the data spanning over seven decades leads to a new general ephemeris formula and the determination of a new value of the updated period 0.072926187( ± 3)d. The available times of maximum light can be well modeled with either a triple linear fit, or a fit concerning a continuously increasing period change combined with the light-time effect of an orbital motion. On the contrary, the decreasing period hypothesis suggested by some previous investigators appears to be rejected. The frequency analyses of the data collected in 2004 and 2006 provide the radial pulsation frequency 13.713 cd-1and its four harmonics. However, although extra powers were present around the reported secondary frequency 17.8 cd-1in the Fourier transformations of the new data sets, its signal-to-noise ratios were too low to detect this frequency.