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Eclipsing binaries in the open cluster Ruprecht 147. II: EPIC 219568666

We report our spectroscopic monitoring of the detached, grazing, and slightly eccentric 12-day double-lined eclipsing binary EPIC 219568666 in the old nearby open cluster Ruprecht 147. This is the second eclipsing system to be analyzed in this cluster, following our earlier study of EPIC 219394517. Our analysis of the radial velocities combined with the light curve from the K2 mission yield absolute masses and radii for EPIC 219568666 of M1 = 1.121 +/- 0.013 M(Sun) and R1 = 1.1779 +/- 0.0070 R(Sun) for the F8 primary, and M2 = 0.7334 +/- 0.0050 M(Sun) and R2 = 0.640 +/- 0.017 R(Sun) for the faint secondary. Comparison with current stellar evolution models calculated for the known metallicity of the cluster points to a primary star that is oversized, as is often seen in active M dwarfs, but this seems rather unlikely for a star of its mass and with a low level of activity. Instead, we suspect a subtle bias in the radius ratio inferred from the photometry, despite our best efforts to avoid it, which may be related to the presence of spots on one or both stars. The radius sum for the binary, which bypasses this possible problem, indicates an age of 2.76 +/- 0.61 Gyr that is in good agreement with a similar estimate from the binary in our earlier study.