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 which are candidates for harboring substellar companions. We have detected the KI and NaI absorption lines of the companion star in VY Aqr. The overall spectral distribution in this system is best fit with a M9.5 type dwarf spectra, implying a distance of $100 \pm 10$ 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 $\mu$m spectral region in short orbital period cataclysmic variables may be an useful indicator of the companion spectral type. The SED fitting for RZ Leo and CU Vel suggests M5 type dwarf companions, and distances of 340 $\pm$ 110 and 150 $\pm$ 50 pc, respectively. These systems may be placed in the upper evolution branch for short period cataclysmic variables.Comment: accepted for publication in MNRAS, 6 pages, 7 figure