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We report 11 yr of spectroscopic monitoring of the M-type asymptotic giantbranch star eta Gem, a semiregular variable and a known spectroscopic binarywith a period of 8.2 yr. We combine our radial velocities with others from theliterature to provide an improved spectroscopic orbital solution giving aperiod of 2979 days, which we then use to predict past times of eclipse. Weexamine archival photometry from amateur variable star observers, and othersources, and find many instances of dimmings that occurred at the right time.This confirms previous indications that the system is eclipsing, and it nowranks among those with the longest known periods. No secondary eclipses areseen. The $\sim$0.4 mag eclipses lasting about 5 months are much too deep to beproduced by a stellar companion. We propose instead that the companion issurrounded by a large disk that is at least 1.5 au in diameter, but is likelylarger. We predict the center of the next eclipse will occur on New Year's day,2029.

Eta Geminorum: An Eclipsing Semiregular Variable Star Orbited by a Companion Surrounded by an Extended Disc