We report on the results from a five-night campaign of high-speed spectroscopy of the 17-minute binary AM Canum Venaticorum , obtained with the 4.2-m William Herschel Telescope on La Palma .

We detect a kinematic feature that appears to be entirely analogous to the ‘ central spike ’ known from the long-period , emission-line AM CVn stars GP Com , V396 Hya and SDSS J124058.03 - 015919.2 , which has been attributed to the accreting white dwarf .
Assuming that the feature indeed represents the projected velocity amplitude and phase of the accreting white dwarf we derive a mass ratio q = 0.18 \pm 0.01 for AM CVn .
This is significantly higher than the value found in previous , less direct measurements .
We discuss the implications for AM CVn ’ s evolutionary history and show that a helium star progenitor scenario is strongly favoured .
We further discuss the implications for the interpretation of AM CVn ’ s superhump behaviour , and for the detectability of its gravitational-wave signal with LISA .

In addition we demonstrate a method for measuring the circularity or eccentricity of AM CVn ’ s accretion disc , using stroboscopic Doppler tomography .
We test the predictions of an eccentric , precessing disc that are based on AM CVn ’ s observed superhump behaviour .
We limit the effective eccentricity in the outermost part of the disc , where the resonances that drive the eccentricity are thought to occur , to e = 0.04 \pm 0.01 , which is smaller than previous models indicated .