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We present an analysis of a slightly eccentric ($e=0.05$), partiallyeclipsing long-period ($P = 69.73$ d) main sequence binary system (WOCS 12009,Sanders 1247) in the benchmark old open cluster M67. Using Kepler K2 andground-based photometry along with a large set of new and reanalyzed spectra,we derived highly precise masses ($1.111\pm0.015$ and $0.748\pm0.005 M_\odot$)and radii ($1.071\pm0.008\pm0.003$ and $0.713\pm0.019\pm0.026 R_\odot$, withstatistical and systematic error estimates) for the stars. The radius of thesecondary star is in agreement with theory. The primary, however, isapproximately $15\%$ smaller than reasonable isochrones for the clusterpredict. Our best explanation is that the primary star was produced from themerger of two stars, as this can also account for the non-detection ofphotospheric lithium and its higher temperature relative to other cluster mainsequence stars at the same $V$ magnitude. To understand the dynamicalcharacteristics (low measured rotational line broadening of the primary starand the low eccentricity of the current binary orbit), we believe that the mostprobable (but not the only) explanation is the tidal evolution of a closebinary within a primordial triple system (possibly after a period ofKozai-Lidov oscillations), leading to merger approximately 1Gyr ago. This starappears to be a future blue straggler that is being revealed as the clusterages and the most massive main sequence stars die out.

The K2 M67 Study: A Curiously Young Star in an Eclipsing Binary in an Old Open Cluster