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OU Vir: post-superoutburst fading stage Dear Colleagues, Tom Krajci reported another set of excellent post-superoutburst data (May 21). The Kyoto team also succeeded in obtaining a full set of data. The object still stays well above the quiescent level, indicating that the object is experiencing the post-superoutburst slow fading stage, which is sometimes most conspicuously seen in low mass-transfer dwarf novae. The meain fading rate is 0.1 mag/d, which is comparable to that of the plateau stage. The most striking example of such a continuing slow fade was seen in WX Cet in 1998 (Kato et al. 2001, PASJ 53, 893), in which the phenomenon was described as: "WX Cet showed a slow fading after the superoutburst. This phenomenon may be an exemplification of a gradual decay of the disk viscosity, which determines the luminosity of the disk. Following Osaki et al. (2001), we suspect that the ignition of thermal instability accidentally failed to occur under competition with the decay of viscosity in the present post-superoutburst state of WX Cet. The post-superoutburst decline of WX Cet was almost perfectly linear (exponential), with a rate of 0.10 mag/d. Interestingly, this rate of decline is almost perfectly identical with the mean rate of decline (0.10 mag/d) during the plateau stage. Although this coincidence may be merely accidental, this may suggest the existence of a time scale for the decay of the disk viscosity related to the decay of the superoutburst plateau." This phenomenon, if it reflects the slow decay of the viscosity, may suggest a near-future (within a few days) rebrightening. If it doesn't occur, there may be a failure of outburst ignition, as observed in WX Cet in 1998. Up to now, the post-superoutburst variation in OU Vir is well represented by a combination of two periods: 0.0727 d and 0.0746 d. The former period corresponds to the orbital period (agrees within estimation errors). The latter period may be related to late superhumps, although the perliminarily measured period is slightly longer than the superhump period. We absolutely need more data (as complete as possible) to exactly follow the period structure and evolution, not to mention eclipses. The OU Vir observations have been extremely productive, and we expect upcoming excellent runs during these dark nights! Regards, Taichi Kato VSNET Collaboration team
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