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 M _ { 1 } = 1.121 \pm 0.013 ~ { } M _ { \sun } and R _ { 1 } = 1.1779 \pm 0.0070 ~ { } R _ { \sun } for the F8 primary , and M _ { 2 } = 0.7334 \pm 0.0050 ~ { } M _ { \sun } and R _ { 2 } = 0.640 \pm 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 \pm 0.61 Gyr that is in good agreement with a similar estimate from the binary in our earlier study .