Accretion disc models for AM Canum Venaticorum systems

Using a model of the vertical structure of accretion discs, we calculate di sc spectra across a range of system parameters for the helium-rich AM CVn c ataclysmic variables, and investigate thermal instabilities in their discs. Applying a chi(2)-minimisation technique, predicted HeI and HI continuum an d line profiles are compared with those observed for two such systems, AM C Vn and CR Boo, to estimate several system parameters. We find: - mean accretor masses close to, but somewhat higher than, the 0.7-0.8M(cir cle dot) range predicted by present population synthesis studies; - typical helium-to-hydrogen number density ratios of 10(2)-10(4), consiste nt with a helium-degenerate donor surrounded by a helium-rich envelope; and - mass-transfer rates of about 10(17)gs(-1), which may place the systems in the region of potential disc instability. S-curves calculated for the best-fit models are consistent with a thermally -stable disc in AM CVn, and a thermally unstable disc in CR Boo.