  We consider the implications of the detection of spiral structure in the
accretion disc of the binary IP Pegasi. We use numerical simulations of the
development of a disc outburst to construct predicted Doppler tomograms, which
are found to be in close agreement with the observations if the spiral pattern
arises as a transient feature when the disc expands viscously at the start of
the outburst. The good agreement of such viscous disc simulations with the data
is consistent with models in which most of the angular momentum transport in
the disc originates in internal stresses rather than globally excited waves or
shocks. Future detailed observations of the development of transient spiral
features offer the potential to measure the dependence of the disc viscosity on
the local physical conditions in the disc.
