Z Cam stars: A particular response to a general phenomenon

We show that the disc instability model can reproduce all the observed prop erties of Z Cam stars if the energy equation includes heating of the outer disc by the mass-transfer stream impact and by tidal torques and if the mas s-transfer rate from the secondary varies by about 30% around the value cri tical for stability In particular the magnitude difference between outburst maxima and standstills corresponds to observations, all outbursts are of t he inside-out type and can be divided into two classes: long (wide) and sho rt (narrow) outbursts, as observed. Mass transfer rate fluctuations should occur in other dwarf novae but one can exclude variations similar to those observed in magnetic systems (AM Her's and some DQ Her's) and some nova-lik e systems (VY Scl's), in which hi become very small during low states: thes e would produce mini-outbursts which, although detectable, have never been observed.