The helium-rich cataclysmic variable SBSS 1108+574

Abstract

We present time-resolved spectroscopy and photometry of the dwarf nova SBSS\n1108+574, obtained during the 2012 outburst. Its quiescent spectrum is\nunusually rich in helium, showing broad, double-peaked emission lines from the\naccretion disc. We measure a line flux ratio HeI 5875/Halpha = 0.81 +/- 0.04, a\nmuch higher ratio than typically observed in cataclysmic variables (CVs). The\noutburst spectrum shows hydrogen and helium in absorption, with weak emission\nof Halpha and HeI 6678, as well as strong HeII emission.\n From our photometry, we find the superhump period to be 56.34 +/- 0.18\nminutes, in agreement with the previously published result. The spectroscopic\nperiod, derived from the radial velocities of the emission lines, is found to\nbe 55.3 +/- 0.8 minutes, consistent with a previously identified photometric\norbital period, and significantly below the normal CV period minimum. This\nindicates that the donor in SBSS 1108+574 is highly evolved. The superhump\nexcess derived from our photometry implies a mass ratio of q = 0.086 +/- 0.014.\nOur spectroscopy reveals a grazing eclipse of the large outbursting disc. As\nthe disc is significantly larger during outburst, it is unlikely that an\neclipse will be detectable in quiescence. The relatively high accretion rate\nimplied by the detection of outbursts, together with the large mass ratio,\nsuggests that SBSS 1108+574 is still evolving towards its period minimum.\n