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X-ray and ultraviolet observations of the eclipsing cataclysmic variables OV Bootis and SDSS J103533.02+055158.3 with degenerate donors

The majority of cataclysmic variables are predicted to be post-period minimum systems with degenerate donor stars, the period bouncers. Owing to their intrinsic faintness, however, only a handful of these systems have so far been securely identified. We want to study the X-ray properties of two eclipsing period bouncers, OV Bootis and SDSS J103533.02+055158.3, that were selected for this study due to their proximity to Earth. We have obtained XMM-Newton phase-resolved X-ray and ultraviolet observations of the two objects for spectral and timing analysis. Owing to a recent dwarf nova outburst OV Boo was much brighter than SDSS J103533.02+055158.3 at X-ray and ultraviolet wavelengths and the eclipse could be studied in some detail. An updated eclipse ephemeris was derived. The X-rays were shown to originate close to the white dwarf, the boundary layer, with significant absorption affecting its spectrum. There was no absorption in SDSS J103533.02+055158.3, despite being observed at the same inclination indicating different shapes of the disk and the disk rim. The white-dwarf temperature was re-determined for both objects: the white dwarf in OV Boo was still hot (23,000 K) five months after a dwarf nova outburst, and the white dwarf in SDSS J103533.02+055158.3 hotter than assumed previously (Teff = 11,500 K). All three cataclysmic variables with degenerate donors studied so far in X-rays, including SDSS J121209.31+013627.7, were clearly discovered in X-rays and revealed mass accretion rates dot(M) >= 8 x 10^(-15) Msun/ yr. If their X-ray behavior is representative of the subpopulation of period bouncers, the all-sky X-ray surveys with eROSITA together with comprehensive follow-up will uncover new objects in sufficient number to address the remaining questions concerning late-stage cataclysmic variable evolution.