  We present an analysis of a slightly eccentric ($e=0.05$), partially
eclipsing long-period ($P = 69.73$ d) main sequence binary system (WOCS 12009,
Sanders 1247) in the benchmark old open cluster M67. Using Kepler K2 and
ground-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 $0.713\pm0.019\pm0.026 R_\odot$, with
statistical and systematic error estimates) for the stars. The radius of the
secondary star is in agreement with theory. The primary, however, is
approximately $15\%$ smaller than reasonable isochrones for the cluster
predict. Our best explanation is that the primary star was produced from the
merger of two stars, as this can also account for the non-detection of
photospheric lithium and its higher temperature relative to other cluster main
sequence stars at the same $V$ magnitude. To understand the dynamical
characteristics (low measured rotational line broadening of the primary star
and the low eccentricity of the current binary orbit), we believe that the most
probable (but not the only) explanation is the tidal evolution of a close
binary within a primordial triple system (possibly after a period of
Kozai-Lidov oscillations), leading to merger approximately 1Gyr ago. This star
appears to be a future blue straggler that is being revealed as the cluster
ages and the most massive main sequence stars die out.
