The Clusters AgeS Experiment (CASE): The Blue Straggler Star M55-V60 Caught Amidst Rapid Mass Exchange

We analyze light and velocity curves of the eclipsing blue straggler V60 in the field of the globular cluster M55. We derive Mp=1.259± 0.025 Msun, Rp=1.102±0.021 Rsun, M bolp=3.03±0.09 mag for the primary and Ms=0.327±0.017 Msun, Rs=1.480±0.011 Rsun, Mbols=4.18±0.12 mag for the secondary. We measure an apparent distance modulus (m-M)V=14.04±0.09 mag. Based on the systemic velocity, distance, and proper motion of V60 we conclude that the system is a member of the cluster and argue that its present state is a result of rapid but conservative mass exchange which the binary is still undergoing. We report a peculiar blue excess on the ascending branch of the primary eclipse of V60 and discuss its possible origin.