that given by McClintock and Remillard (1986), but within the observational uncertainty (see the discussion in McClintock et al. 1995). The transferred mass from the companion is initially accumulated in the outermost disk ($6{\times}10^{10}-10^{11}$ cm) and then diffuses and accretes inward.

The outburst recurrence time in these models is about 3–4 years. This is too short for most observed systems, but does not affect the qualitative thermodynamics of the irradiation, which is the current focus. The recurrence time would be longer if the transfer rate were smaller (see also King et al. 1997). For this relatively short recurrence time, the inner disk does not have time to return entirely to the cool state. The mass-flow rate in the inner disk, and hence the generated radiation, does drop by many orders of magnitude (about 7) from the peak, which is sufficient to capture the essence of the variable production of irradiation and how it reacts with the outer disk during the phases of heating and cooling.

Given the limitation of the small number of zones, we have checked the basic behavior of the code by computing the speed of propagation of the cooling wave. Cannizzo et al. (1995) express the speed 