Orbital Periods for the Unusual Dwarf Novae ER Ursae Majoris and V1159 Orionis

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Abstract

ER UMa (PG 0943+521), V1159 Ori, and RZ LMi constitute a small recently- recognized group of dwarf novae, called the RZ LMi stars or the ER UMa stars. They share many features of the SU UMa-type, but have shorter outburst cycles than had been known previously. Here we establish orbital periods, based on emission-line radial velocities, for ER UMa (0.06366 ± 0.00003 d = 91.67 ± 0.04 min) and V1159 Ori (0.06217801 ± 1.3 × 10-7 d = 89.5363 ± 0.0002 min). The precise orbital period for V1159 Ori depends on an extrapolation of cycle count from 1991 to 1994, which is slightly uncertain, but the daily cycle count is reasonably secure. These orbital periods are similar to those of the more common, less frequently outbursting SU UMa stars. As is generally the case, the orbital periods derived here are slightly shorter than the periods of photometric superhumps which appear in superoutburst. The superhump period excesses found here are compatible with, but marginally larger than, those seen in other SU UMa stars of similar period. While the empirical link between ER UMa stars and SU UMa stars is generally strengthened by these results, the trend toward slightly larger period excesses (if true) can be interepreted as a tendency for systems with larger mass ratios to develop larger mass-transfer rates at a given orbital period.

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