2000AJ....119..890B

We describe optical photometric and spectroscopic observations of the bright variable BG Geminorum. Optical photometry shows a pronounced ellipsoidal variation of the K0 I secondary, with amplitudes of ∼0.5 mag at VR_CI_C and a period of 91.645 days. A deep primary eclipse is visible for λ≲4400 Å; a shallower secondary eclipse is present at longer wavelengths. Eclipse timings and the radial velocity curve of the K0 secondary star indicate an interacting binary where a lobe-filling secondary, M₂∼0.5 M_☉, transfers material into a extended disk around a massive primary, M₁∼4.5 M_☉. The primary star is either an early B-type star or a black hole. If it did contain a black hole, BG Gem would be the longest period black hole binary known by a factor of 10, as well as the only eclipsing black hole binary system.