Low-frequency variability and binarity of the delta Scuti star XX Pyx

We analyse 147 h of single-site CCD time series photometry of the multiperi odic low-amplitude delta Scuti star XX Pyx with the aim of investigating va riability at low frequencies. Part of the data were obtained in the context of the 1998 multisite campaign on XX Pyx, the results of which were descri bed by Handler et al. We find that periodic low-frequency variations are pr esent in the XX Pyx light curves, and we detect two frequencies at f(A) = 0 .8695 cycle d(-1) and f(B) = 1.7352 cycle d(-1), respectively, with amplitu des of 4.5 and 5.4 mmag. The low-frequency variability is intrinsic to XX P yx, and cannot be attributed to instrumental or atmospheric effects. The ne ar 2:1 ratio of the frequencies leads us to suggest that XX Pyx is a delta Scuti star in a binary system, with a possible binary period of 27.6 h. Thi s is strongly supported by the detection of radial velocity variations from the re-analysis of echelle spectra obtained by Handler et al. However, in the absence of a spectroscopic period, alternative explanations of the phot ometric variability involving pulsation are also possible; the variations o ccur close to possible combination frequencies of the short-period (delta S cuti) variations, but high Q values of 0.57 and 0.28 d suggest that the var iations are not a result of normal (p-)modes. They could possibly be due to g-modes excited to observable amplitudes as a result of resonance effects. Surface features (spots) are unlikely to be the cause of the variations. We searched for combination frequencies (f(i) - f(j)), f being the normal d elta Scuti frequencies detected by Handler et al., but failed to detect any .