We examine the AAVSO light curve of U Geminorum from 1908 to 2002 , with particular focus on the October 1985 outburst .
This outburst was longer than any other seen in U Gem by about a factor of 2 , and appears to be unique among all dwarf nova outbursts seen in systems with orbital periods longer than 3 hr in that one can measure the decay time scale during the initial slow decay .
This rate is \sim 26 \pm 6 d mag ^ { -1 } .
Using estimates of the rate of accretion during outburst taken from Froning et al. , one can show that \sim 10 ^ { 24 } g of gas was accreted onto the white dwarf during the outburst which constrains the surface density in the outer accretion disk to be \sim 600 g cm ^ { -2 } .
The observed time scale for decay is consistent with that expected in U Gem , given its orbital period and disk mass at the time the outburst began .
The data are not of sufficient quality to be able to ascertain a deviation from exponentiality in the decay light curve ( as in the SU Ursa Majoris stars ’ superoutbursts ) .
When coupled with the viscous time inferred from the ( short orbital period ) SU UMa stars , the U Gem viscous time scale lends support to the standard model in which the decays in dwarf novae can either be viscous or thermal , with the ratio between them being roughly h / r where h is the vertical pressure scale height in the disk .
Indeed , dwarf novae are the only systems for which one can be reasonably certain of the identification of “ viscous ” and “ thermal ” decays .