  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 of 0.036. We compare both the average and time-resolved spectra of 'SN
2003aw' and SDSS J124058.03-015919.2. 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 'SN 2003aw'. 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.

Astrophysics