  A thermal/viscous instability of the accretion disc arising when hydrogen is
partially ionized is the most popular explanation of the outbursts of both
dwarf novae (DN) and soft X-ray transients (SXT). During an outburst, the disc
expands and may reach the 3:1 resonance radius, triggering the so-called tidal
instability. The consequences of such an instability are still debated; it is
generally accepted that it results in the observed periodic modulation in SU
UMa systems, and to large amplitude outbursts in these systems, the latter
being more controversial. We discuss both the theoretical and observational
evidences for such radius variations in DNs and SXTs, and the consequences of
the application of the tidal instability in SXT.
