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 P _ { \mathrm { V 803 Cen } } = 1596.4 \pm 1.2 \mathrm { s } of the S-wave feature , which is significantly shorter than the 1611-second 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 \sim 0.07 rather than q \sim 0.016 .
This relaxes the constraints on the masses of the components considerably : the donor star does then 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 P _ { \mathrm { HP Lib } } = 1102.8 \pm 0.2 \mathrm { s } .
This supports the identification of HP Lib ’ s photometric periods found in the literature , and the constraints upon the masses derived from them .