Phase - resolved spectroscopy of the helium dwarf nova ‘ SN 2003 aw ’ in quiescence

High time resolution spectroscopic observations of the ultra-compact helium dwarf nova ‘SN 2003aw’ in its quiescent state at V ∼ 20.5 reveal its orbital period at 2027.8±0.5 seconds or 33.80 minutes. Together with the photometric “superhump” period of 2041.5±0.5 seconds, this implies a mass ratio q≈ 0.036. We compare both the average and time-resolved spectra of ‘SN 2003aw’ and SDSS J124058.03−015919.2 (Roelofs et al. 2005). Both show a DB white dwarf spectrum plus an optically thin, helium-dominated accretion disc. ‘SN 2003aw’ distinguishes itself from the SDSS source by its strong calcium H & K emission lines, suggesting higher abundances of heavy metals than the SDSS source. The silicon and iron emission lines observed in the SDSS source are about twice as strong in ‘SN2003aw’. The peculiar “double bright spot” accretion disc feature seen in the SDSS source is also present in time-resolved spectra of ‘SN 2003aw’, albeit much weaker.