The helium-rich cataclysmic variable SBSS 1108+574

TL;DR

This study analyzes the helium-rich dwarf nova SBSS 1108+574 during outburst, revealing a highly evolved donor star, a short orbital period below the CV minimum, and a large accretion disc with grazing eclipse features.

Contribution

First detailed spectroscopic and photometric analysis of SBSS 1108+574 showing its unique helium-rich composition and evolved donor star, indicating its evolutionary status towards the period minimum.

Findings

Helium line flux ratio HeI 5875/Halpha is unusually high at 0.81.

Orbital period is 55.3 minutes, below the typical CV period minimum.

Mass ratio q is approximately 0.086, indicating a highly evolved donor.

Abstract

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