Abstract
Mean data of WZ Sge and SU UMa stars are given in Table 1. Typical number of stars in each sample was 5 and 15 respectively. The most significant parameter to distinguish between WZ Sge and SU UMa stars is ΔT/T rec , as expected from the observational definition of WZ Sge stars. Significant differences are observed also in the mean P orb , M 2/M 1 and W α values, whereas UBV colors are similar. In addition, we have compared ΔT/T rec with the orbital period in a sample of 4 WZ Sge, 6 SU UMa and 10 U Gem subtype dwarf novae. WZ Sge and SU UMa stars show a steep correlation: T/T rec = (-0.74 ± 0.14) + (0.57 ± 0.09)P orb (h), with correlation coefficient 0.91 and σ = 0.07. In addition, when comparing the general tendency with theoretical models (e.g. Cannizzo et al. 1988, ApJ 333, 227), we have found that WZ Sge stars have very low viscosity disks during quiescence (a ~ 0.0001) but disks with normal viscosities during eruptions (a ~ 0.1 - 1). Table 1 shows color averages calculed from the Bruch and Engel (1994 A&AS 104, 79) UBV catalogue. Equivalent widths are from references listed in Mennickent (1994, A&A 285, 979) and from Mennickent (unpublished). Orbital periods were obtained from Molnar and Kobulnicky (1992, ApJ 392, 678), Leibowitz et al. (1994, ApJ 421, 771) and from Mennickent (this conference and unpublished). The outburst duration (ΔT) and the recurrence times (T rec ) were compiled from several data papers available in the literature.
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