Evidence for the Keplerian orbit of a close companion around a giant star

Close companions influence stellar evolution through tidal interactions, mass transfer, and mass loss effects. While such companions are detected around young stellar objects, main- sequence stars, red giants, and compact objects, direct observational evidence of close-in companions around asymptotic giant branch (AGB) stars has remained elusive. Here, we present (sub)millimeter time-domain imaging spectroscopy revealing a close-in companion in Keplerian motion around the AGB star π 1 Gruis. The companion, slightly more mas- sive than the AGB star, is likely a main-sequence star. Unlike more evolved stars with com- panions at comparable distances, π1 Gru’s companion follows a circular orbit, suggesting an eccentricity-generating mechanism late- or post-AGB. Our analysis suggests that model- predicted circularization rates may be underestimated. Our results highlight the potential of (sub)millimeter interferometry in detecting close companions to giant stars and open new avenues for our understanding of tidal interaction physics and binary evolution.