Infrared spectroscopy of cataclysmic variables - III. Dwarf novae below the period gap and nova-like variables

Abstract

We present K-band spectra of the short-period dwarf novae YZ Cnc, LY Hya, BK Lyn, T Leo, SW UMa and WZ Sge, the nova-like variables DW UMa, V1315 Aql, RW Tri, VY Scl, UU Aqr and GP Com, and a series of field dwarf stars with spectral types ranging from K2 to M6. The spectra of the dwarf novae are dominated by emission lines of H I and He I. The large velocity and equivalent widths of these lines, in conjunction with the fact that the lines are double-peaked in the highest inclination systems, indicate an accretion disc origin. In the case of YZ Cnc and T Leo, for which we obtained time-resolved data covering a complete orbital cycle, the emission lines show modulations in their equivalent widths that are most probably associated with the bright spot (the region where the gas stream collides with the accretion disc). There are no clear detections of the secondary star in any of the dwarf novae below the period gap, yielding upper limits of 10-30 per cent for the contribution of the secondary star to the observed K-band flux. In conjunction with the K-band magnitudes of the dwarf novae, we use the derived secondary star contributions to calculate lower limits to the distances to these systems. The spectra of the nova-like variables are dominated by broad, single-peaked emission lines of H 1 and He 1 - even the eclipsing systems we observed do not show the double-peaked profiles predicted by standard accretion disc theory. With the exception of RW Tri, which exhibits Na 1, Ca 1 and 12CO absorption features consistent with a M0V secondary contributing 65 per cent of the observed K-band flux, we find no evidence for the secondary star in any of the nova-like variables. The implications of this result are discussed.