Abstract
We calculate the heating and cooling of the accreting white dwarf (WD) in the ultracompact AM Canum Venaticorum (AM CVn) binaries and show that the WD can contribute significantly to their optical and ultraviolet emission. We estimate the WD's effective temperature, Teff, using the optical continuum for a number of observed binaries, and we show that it agrees well with our theoretical calculations. Driven by gravitational radiation losses, the time-averaged accretion rate, <M˙>, decreases monotonically with increasing Porb, covering 6 orders of magnitude. If the short-period (P<SUB>orb</SUB><10 minutes) systems accrete at a rate consistent with gravitational radiation via direct impact, we predict their unpulsed optical/UV light to be that of the T<SUB>eff</SUB>>50,000 K accreting WD. At longer Porb we calculate the Teff and absolute visual magnitude, M<SUB>V</SUB>, that the accreting WD will have during low accretion states, and we find that the
