  The interacting binary white dwarf (AM CVn) systems HM Cnc and V407 have
orbital periods of 5.4 min and 9.5 min, respectively. The two systems are
characterized by an "on/off" behaviour in the X-ray light curve, and optical
light curves that are nearly sinusoidal and which lead the X-ray light curves
in phase by about 0.2 in both systems. Of the models that have been proposed to
explain the observations, the one that seems to require the least fine tuning
is the direct impact model of Marsh & Steeghs (2002). In this model, the white
dwarf primary is large enough relative to the semi-major axis that the
accretion stream impacts the surface of the primary white dwarf directly
without forming an accretion disc. Marsh & Steeghs proposed that in this
situation there could be a flow set up around the equator with a decreasing
surface temperature the further one measured from the impact point. In this
study, we estimate the light curves that might result from such a temperature
distribution, and find them to be reasonable approximations to the
observations. One unexpected result is that two distinct X-ray spots must exist
to match the shape of the X-ray light curves.
