On the orbital periods of the AM CVn stars HP Librae and V803 Centauri

Contains fulltext : 35257.pdf ( ) (Open Access)We analyse high time resolution spectroscopy of the AM CVn stars HP Librae and V803 Centauri, taken with the New Technology Telescope (NTT) and the Very Large Telescope (VLT) of the European Southern Observatory, Chile. We present evidence that the literature value for V803 Cen's orbital period is incorrect, based on an observed `S-wave' in the binary's spectrogram. We measure a spectroscopic period PV803Cen = 1596.4 +/- 1.2s of the S-wave feature, which is significantly shorter than the 1611-s periods found in previous photometric studies. We conclude that the latter period likely represents a `superhump'. If one assumes that our S-wave period is the orbital period, V803 Cen's mass ratio can be expected to be much less extreme than previously thought, at q ~ 0.07 rather than q ~ 0.016. This relaxes the constraints on the masses of the components considerably: the donor star then does not need to be fully degenerate, and the mass of the accreting white dwarf no longer has to be very close to the Chandrasekhar limit. For HP Lib, we similarly measure a spectroscopic period PHPLib = 1102.8 +/- 0.2s. This supports the identification o