  With orbital periods of the order of tens of minutes or less, the AM Canum
Venaticorum stars are ultracompact, hydrogen deficient binaries with the
shortest periods of any binary subclass, and are expected to be among the
strongest gravitational wave sources in the sky. To date, the only known
eclipsing source of this type is the P = 28 min binary SDSS J0926+3624. We
present multiband, high time resolution light curves of this system, collected
with WHT/ULTRACAM in 2006 and 2009. We supplement these data with additional
observations made with LT/RISE, XMM_Newton and the Catalina Real-Time Transient
Survey. From light curve models we determine the mass ratio to be q = M2 / M1 =
0.041 +/- 0.002 and the inclination to be 82.6 +/- 0.3 deg. We calculate the
mass of the primary white dwarf to be 0.85 +/- 0.04 solar masses and the donor
to be 0.035 +/- 0.003 solar masses, implying a partially degenerate state for
this component. We observe superhump variations that are characteristic of an
elliptical, precessing accretion disc. Our determination of the superhump
period excess is in agreement with the established relationship between this
parameter and the mass ratio, and is the most precise calibration of this
relationship at low q. We also observe a quasi-periodic oscillation in the 2006
data, and we examine the outbursting behaviour of the system over a 4.5 year
period.
