  We propose a simple explanation for the constant mean brightness observed
between outbursts in dwarf novae. Secular changes in the total energy
dissipation rate of the accretion disc brought about by variations in surface
density, temperature and disc radius can be regulated by the gradual cooling of
a small, critically-stable hot inner region. The hypothesis is supported by
two-dimensional time-dependent numerical models of dwarf nova accretion discs.
