Binarity and Accretion in AGB Stars: HST/STIS Observations of UV Flickering in Y Gem.

Binarity is believed to dramatically affect the history and geometry of mass loss in AGB and post-AGB stars, but observational evidence of binarity is sorely lacking. As part of a project to search for hot binary companions to cool AGB stars using the GALEX archive, we discovered a late-M star, Y Gem, to be a source of strong and variable UV and X-ray emission. Here we report UV spectroscopic observations of Y Gem obtained with the Hubble Space Telescope that show strong flickering in the UV continuum on time-scales of 5e-7 Msun/yr; we infer that Roche lobe overflow is the most likely binary accretion mode for Y Gem.