* Astronomy

Title: ULTRACAM observations of SDSS J170213.26+322954.1 - an eclipsing cataclysmic variable in the period gap
Authors: S. P. Littlefair, V. S. Dhillon, T. R. Marsh, B. T. Gaensicke

Researchers present high-speed, three-colour photometry of the eclipsing cataclysmic variable SDSS J170213.26+322954.1 (hereafter SDSS J1702+3229). This system has an orbital period of 2.4 hours, placing it within the ''period gap'' for cataclysmic variables.
They determine the system parameters via a parameterised model of the eclipse fitted to the observed light curve by chi-squared minimisation. They obtain a mass ratio of q = 0.215 ± 0.015 and an orbital inclination i = 82.4 ± 0.4 degrees. The primary mass is M_w = 0.94 ± 0.01 solar masses. The secondary mass and radius are found to be Mr = 0.20 ± 0.01 solar masses and Rr = 0.243 ± 0.013 solar radii respectively. They 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 ± 0.5V). 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.

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