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 \pm 0.015 and 0.748 \pm 0.005 M _ { \odot } ) and radii ( 1.071 \pm 0.008 \pm 0.003 and 0.713 \pm 0.019 \pm 0.026 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 1 Gyr 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 .