Amplitude and Period Changes in the SX Phoenicis Star XX Cygni

The new time-resolved CCD photometry of XX Cyg was presented. We investigate the time-dependent behaviour of the light variations by reanalyzing the data in the literature. XX Cyg was demonstrated beyond any doubt again to be a monoperiodic pulsator. Its light curves can be well reproduced by the sinusoids with the fundamental frequency f^sub 0^ = 7.41481 ± 0.00001 cycle d^sup -1^(P^sub 0^ = 0^sup d^^sub .^ 1348652 ± 0.0000001) and its first nine harmonics. Fourier, wavelet and O-C analyses based on all the available data reveals the amplitude and period variations during the years 1974 to 2000.The general trend of the amplitude changes is decreasing at a rate of one percent milli-magnitudes per year. The parabolic analysis on the updated 101 times of maximum light suggests a slowly continuous increasing at a rate of 1.3 × 10^sup -3^ days per million years in the period.[PUBLICATION ABSTRACT]