The Dwarf Novae of Shortest Period

We present observations of the dwarf novae GW Lib, V844 Her, and DI UMa.Radial velocities of H-alph yield orbital periods of 0.05332 +- 0.00002 d (=76.78 m) for GW Lib and and 0.054643 +- 0.7 d (= 78.69 m) for V844 Her.Recently, the orbital period of DI UMa was found to be only 0.054564 +-0.2 d (= 78.57 m) by Fried et al. (1999), so these are the three shortestorbital periods among dwarf novae with normal-abundance secondaries. GW Lib has attracted attention as a cataclysmic binary showing apparent ZZCeti-type pulsations of the white dwarf primary. Its spectrum shows sharpBalmer emission flanked by strong, broad Balmer absorption, indicating adominant contribution by white-dwarf light. Analysis of the Balmer absorptionprofiles is complicated by the unknown residual accretion luminosity and lackof coverage of the high Balmer lines. Our best-fit model atmospheres aremarginally hotter than the ZZ Ceti instability strip, in rough agreement withrecent ultraviolet results from HST. The spectrum and outburst behavior of GWLib make it a near twin of WZ Sge, and we estimate it to have a quiescent Vabsolute magnitude 12. Comparison with archival data reveals proper motion of65 +- 12 mas/yr. The mean spectrum of V844 Her is typical of SU UMa dwarf novae. We detectedsuperhumps in the 1997 May superoutburst with superhump period = 0.05597 +-0.00005 d. The spectrum of DI UMa appears normal for a dwarf nova near minimumlight. These three dwarf novae have nearly identical short periods but completelydissimilar outburst characteristics. We discuss possible implications.Comment: Accepted for publication in Publications of the Astronomical Society of the Pacific; 16 pages, 6 figure