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CSS J134052.0+151341 : A Cataclysmic Binary Star with a Stripped, Evolved Secondary

I report spectroscopy and time-series photometry of the cataclysmic binary CSS J134052.0+151341. The optical light is dominated by the secondary star, which I classify as K4 (+-2 subclasses), yet the orbital period derived from the absorption radial velocities is only 2.45 hr, implying a Roche radius much too small to contain a main-sequence K star. The spectrum shows enhanced sodium absorption in several lines, suggesting that the surface material has been processed at high temperatures. CSS J134052.0+151341 appears to be a rare example of a cataclysmic binary in which the secondary star is the stripped core of a formerly much more massive star, that began mass transfer after much of the core's nuclear evolution had taken place.