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 \sim 0.5 mag at VR _ { C } I _ { C } and a period of 91.645 days .
A deep primary eclipse is visible for \lambda \lesssim 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 , M _ { 2 } \sim 0.5 ~ { } { M _ { \odot } } , transfers material into a extended disk around a massive primary , M _ { 1 } \sim 4.5 ~ { } { M _ { \odot } } .
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 .