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 M-p = 1.259 +/- 0.025 M-circle dot, R-p = 1.102 +/- 0.021 R-circle dot, M-p(bol) = 3.03 +/- 0.09 mag for the primary and M-s = 0.327 +/- 0.017 M-circle dot, R-s = 1.480 +/- 0.011 R-circle dot, M-s(bol) = 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.