A J-band detection of the donor star in the dwarf nova OY Carinae and an optical detection of its ’iron curtain’

Purely photometric models can be used to determine the binary parameters of eclipsing cataclysmic variables (CVs) with a high degree of precision. However, the photometric method relies on a number of assumptions, and to date there have been very few independent checks of this method in the literature. We present time-resolved spectroscopy of the P= 90.9 min eclipsing CV OY Carinae obtained with X-shooter on the Very Large Telescope (VLT), in which we detect the donor star from K I lines in the J band. We measure the radial velocity amplitude of the donor star K2= 470.0 ± 2.7 km s−1, consistent with predictions based upon the photometric method (470 ± 7 km s−1). Additionally, the spectra obtained in the UVB arm of X-shooter show a series of Fe I and Fe II lines with a phase and velocity consistent with an origin in the accretion disc. This is the first unambiguous detection at optical wavelengths of the ‘iron curtain’ of disc material which has been previously reported to veil the white dwarf in this system. The velocities of these lines do not track the white dwarf, reflecting a distortion of the outer disc that we see also in Doppler images. This is evidence for considerable radial motion in the outer disc, at up to 90 km s−1 towards and away from the white dwarf.