The dwarf novae of shortest period

We present observations of the dwarf novae GW Lib, V844 Her, and DI UMa. Radial velocities of Hα yield orbital periods of 0.05332 ± 0.00002 days ( = 76.78 minutes) for GW Lib and 0.054643 ± 0.000007 days ( = 78.69 minutes) for V844 Her. Recently, the orbital period of DI UMa was found to be only 0.054564 ±1 0.000002 days ( = 78.57 minutes) by Fried et al., so these are the three shortest orbital periods among dwarf novae with normal-abundance secondaries. GW Lib has attracted attention as a cataclysmic binary showing apparent ZZ Ceti type pulsations of the white dwarf primary. Its spectrum shows sharp Balmer emission flanked by strong, broad Balmer absorption, indicating a dominant contribution by white dwarf light. Analysis of the Balmer absorption profiles is complicated by the unknown residual accretion luminosity and lack of coverage of the high Balmer lines. Our best-fit model atmospheres are marginally hotter than the ZZ Ceti instability strip, in rough agreement with recent ultraviolet results from the Hubble Space Telescope. The spectrum and outburst behavior of GW Lib make it a near twin of WZ Sge, and we estimate it to have a quiescent M_v ∼ 12. Comparison with archival data reveals proper motion of 65 ± 12 mas yr^-1. The mean spectrum of V844 Her is typical of SU UMa dwarf novae. We detected superhumps in the 1997 May superoutburst with P_sh = 0.05597 ∼ 0.00005 days. The spectrum of DI UMa appears normal for a dwarf nova near minimum light. These three dwarf novae have nearly identical short periods but completely dissimilar outburst characteristics. We discuss possible implications.