ULTRACAM observations of SDSS J170213.26+322954.1 -: an eclipsing cataclysmic variable in the period gap

We present high-speed, three-colour photometry of the eclipsing cataclysmic variable (CV) Sloan Digital Sky Survey J170213.26 + 322954.1 (hereafter SDSS J1702 + 3229). This system has an orbital period of 2.4 h, placing it within the 'period gap' for CVs. We determine the system parameters via a parametrized model of the eclipse fitted to the observed light curve by chi(2) minimization. We obtain a mass ratio of q = 0.215 +/- 0.015 and an orbital inclination i = 82.4 +/- 0.4. The primary mass is M-w = 0.94 +/- 0.01 M-circle dot. The secondary mass and radius are found to be M-r= 0.20 +/- 0.01 M-circle dot and R-r = 0.243 +/- 0.013 R-circle dot, respectively. We find a distance to the system of 440 +/- 30 pc, and an effective temperature for the secondary star of 3800 +/- 100 K (corresponding to a spectral type of M0 +/- 0.5 V). Both the distance and effective temperature are consistent with previous values derived via spectroscopy of the red star. The secondary star is significantly less massive than expected for the orbital period, and significantly warmer than expected for its mass. This can be explained if the secondary star is significantly evolved: the mass and effective temperature are consistent with a secondary star that began mass transfer with a greatly reduced central hydrogen fraction. The nature of the secondary star in SDSS J1702 + 3229 supports predictions that CVs with evolved secondary stars might be found accreting within the period gap.