On The Orbital Period Of The Magnetic Cataclysmic Variable Hu Aquarii

We present an analysis of ULTRACAM light curves of the magnetic cataclysmic variable HU Aquarii which were taken at the VLT in May 2005. Since the light curves were serendipitously obtained during a low state, they allowed us to determine the binary and the stellar parameters with high accuracy. The light curve was decomposed into the components originating from the accretion spot the photosphere surrounding it and the white dwarf itself, which allowed us to extract the eclipse light curve for the pure white dwarf. Combined with high-time resolution observations with different instruments over a 12 year baseline it was possible to get exact eclipse timings of the white dwarf and thus establish a significant deviation from a linear ephemeris. If described by a quadratic term, the period decreases by -1.13 x 10(-11) ss(-1). Interpreting this change in period as a pure angular momentum loss (AML) effect the rate of J = -4.9 x 10(35) erg is much too high to be explained by gravitational radiation alone.