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 +- 0.000007 d (= 78.69 m) for V844 Her.
Recently, the orbital period of DI UMa was found to be only 0.054564 +-
0.000002 d (= 78.57 m) by Fried et al. (1999), 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 HST. The spectrum and outburst behavior of GW
Lib make it a near twin of WZ Sge, and we estimate it to have a quiescent V
absolute magnitude 12. Comparison with archival data reveals proper motion of
65 +- 12 mas/yr.
The mean spectrum of V844 Her is typical of SU UMa dwarf novae. We detected
superhumps 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 minimum
light.
These three dwarf novae have nearly identical short periods but completely
dissimilar outburst characteristics. We discuss possible implications.Comment: Accepted for publication in Publications of the Astronomical Society
of the Pacific; 16 pages, 6 figure