SDSS J0926+3624: the shortest period eclipsing binary star

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 William Herschel Telescope (WHT)/ULTRACAM in 2006 and 2009. We supplement these data with additional observations made with Liverpool Telescope/Rapid Imager to Search for Exoplanets (LT/RISE), XMM-Newton and the Catalina Real-Time Transient Survey. From light curve models we determine the mass ratio to be q = M-2/M-1 = 0.041 +/- 0.002 and the inclination to be mu. We calculate the mass of the primary white dwarf to be 0.85 +/- 0.04 M-circle dot and the donor to be 0.035 +/- 0.003 M-circle dot, 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.