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P. J. Benson, K. K. McLeod, A. Bonanos (Wellesley C.), A. Dullighan (Swarthmore C.), S. J. Kenyon (CfA)
Optical and spectrosopic observations of the bright ellipsoidal eclipsing binary system BG Geminorum indicate a period of 91.645 days. A deep primary eclipse is visible in the spectra for short wavelengths and a shallow secondary eclipse at longer wavelengths. Absorption lines indicate the presence of a K0 supergiant which is orbiting the center of mass at about 75 km/s. Double-peaked H-Balmer lines from an accretion disk have an almost constant velocity outside of primary eclipse, but show a characteristic Rossiter effect consistent with the eclipse of a rapidly rotating disk. Eclipse timings and the radial velocity curve of the lobe filling K0 secondary star indicate a massive primary of about 4.5 solar masses, although we cannot see any spectral lines from the primary. Additional observations are needed to determine whether the primary could be a B-type star or a black hole. If the primary is a black hole, it would be the only eclipsing black hole binary system known and would have the longest known orbital period known for a black hole binary by a factor of about 10.
We thank P. Berlind, M. Calkins, and the observers at the FLWO 60" telescope for acquiring the FAST spectra, S. Tokarz for preliminary reductions of the spectra, and Wellesley College undergraduates for imaging of BG Gem. We also thank NSF grant AST9417359, Wellesley College Brachman Hoffman grants, and the W. M. Keck Foundation for support of this project.