η Geminorum: an eclipsing semiregular variable star orbited by a companion surrounded by an extended disc

Abstract

ABSTRACT We report 11 yr of spectroscopic monitoring of the M-type asymptotic giant branch star η Gem, a semiregular variable and a known spectroscopic binary with a period of 8.2 yr. We combine our radial velocities with others from the literature to provide an improved spectroscopic orbital solution, giving a period of 2979 d, which we then use to predict past times of eclipse. We examine archival photometry from amateur variable star observers, and other sources, and find many instances of dimmings that occurred at the right time. This confirms previous indications that the system is eclipsing, and it now ranks among those with the longest known periods. No secondary eclipses are seen. The ∼0.4 mag eclipses that last about 5 months are much too deep to be produced by a stellar companion. We propose instead that the companion is surrounded by a large disc that is at least 1.5 au in diameter but is likely larger. We predict the centre of the next eclipse will occur on New Year’s Day, 2029.