Theδ Scuti star XX Pyx is an ellipsoidal variable

We present spectroscopic and multicolour photometric evidence supporting the binary nature of the δ Scuti star XX Pyx (CD–24 7599). Applying a cross-correlation technique to the spectra, we find clear radial-velocity variations with a large amplitude. We derive the orbital parameters and confirm an orbital period of 1.15 d, as suggested previously on the basis of photometric variations. The amplitude of the slow variations present in our new multicolour data is wavelength independent, pointing also to a geometric effect as origin of the variability. They are thus fully consistent with the spectral variations and are interpreted as ellipsoidal variations. XX Pyx has a circular orbit of which the radial-velocity variations have a semi-amplitude of 17.8±0.4 km s^-1.The single-lined binary nature of the star, together with the mass function, lead us to conclude that the orbital inclination must be larger than 10°. The orbital solution is compatible with a synchronized M3V companion. The deformation of the primary due to tidal forces is very probably the reason for the failure of detailed seismic modelling efforts done recently.