AM CVn binaries are a class of ultracompact , hydrogen-deficient binaries , each consisting of a white dwarf accreting helium-dominated material from a degenerate or semi-degenerate donor star .
Of the 56 known systems , only Gaia14aae undergoes complete eclipses of its central white dwarf , allowing the parameters of its stellar components to be tightly constrained .
Here , we present phase-resolved optical spectroscopy of Gaia14aae .
We use the spectra to test the assumption that the narrow emission feature known as the ‘ central spike ’ traces the motion of the central white dwarf .
We measure a central spike velocity amplitude of 13.8 \pm 3.2 km/s , which agrees at the 1 \sigma level with the predicted value of 17.6 \pm 1.0 km/s based on eclipse-derived system parameters .
The orbital phase offset of the central spike from its expected position is 4 \pm 15 ^ { \circ } , consistent with 0 ^ { \circ } .
Doppler maps of the \ion HeI lines in Gaia14aae show two accretion disc bright spots , as seen in many AM CVn systems .
The formation mechanism for the second spot remains unclear .
We detect no hydrogen in the system , but we estimate a 3 \sigma limit on H \alpha emission with an equivalent width of -1.14 Å .
Our detection of nitrogen and oxygen with no corresponding detection of carbon , in conjunction with evidence from recent studies , mildly favours a formation channel in which Gaia14aae is descended from a cataclysmic variable with a significantly evolved donor .