  Dwarf nova outbursts result from enhanced mass transport through the
accretion disc of a cataclysmic variable system.
  We assess the question of whether these outbursts are caused by an enhanced
mass transfer from the late-type main sequence star onto the white dwarf
(so-called mass transfer instability model, MTI) or by a thermal instability in
the accretion disc (disc instability model, DIM).
  We compute non-LTE models and spectra of accretion discs in quiescence and
outburst and construct spectral time sequences for discs over a complete
outburst cycle. We then compare our spectra to published optical spectroscopy
of the dwarf nova SS Cygni. In particular, we investigate the hydrogen and
helium line profiles that are turning from emission into absorption during the
rise to outburst.
  The evolution of the hydrogen and helium line profiles during the rise to
outburst and decline clearly favour the disc-instability model. Our spectral
model sequences allow us to distinguish inside-out and outside-in moving
heating waves in the disc of SS Cygni, which can be related to symmetric and
asymmetric outburst light curves, respectively.
