Dwarf nova outbursts. III. The viscosity parameter alpha

Visual light curves based on dwarf nova models, covering a broad range of r elevant parameters, are used to determine the widths of outbursts, rates of decline, and UV delays, as well as their dependence on the orbital period. From a comparison with the observational data it is found that the value o f alpha on the hot branch of the Sigma - T-e relation is alpha(hot) approxi mate to 0.2. An important test is also made concerning the tidal radii of disks. A compa rison of the absolute visual magnitudes at maximum, obtained from model lig ht curves, with the observed M-V(max) gives R-tid/R-Roche approximate to 0. 9, which is the standard value used in model calculations.