A search for brown dwarf like secondaries in cataclysmic variables

Abstract

We present VTL/ISAAC infrared spectroscopy of a sample of short-orbital-period cataclysmic variables that are candidates for harbouring substellar companions. We have detected the K i and Na i absorption lines of the companion star in VY Aqr. The overall spectral distribution in this system is best fitted with an M9.5 type dwarf spectrum, implying a distance of 100 ± 10 pc. VY Aqr seems to fall far from the theoretical distribution of secondary star temperatures around the orbital period minimum. Fitting of the IR spectral energy distribution (SED) was performed by comparing the observed spectrum with late-type templates. The application of such a spectral fitting procedure suggests that the continuum shape in the 1.1–2.5 μm spectral region in short-orbital-period cataclysmic variables may be a useful indicator of the companion's spectral type. SED fitting for RZ Leo and CU Vel suggests M5-type dwarf companions, and distances of 340 ± 110 and 150 ± 50 pc, respectively. These systems may be placed in the upper evolution branch for short-period cataclysmic variables.