An Optical Study of BG Geminorum: An Ellipsoidal Binary with an Unseen Primary Star

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 VRCIC 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.