Eclipsing binaries in star clusters offer more stringent tests of stellar evolution theory than field binaries because models must not only match the binary properties , but also the radiative properties of all other cluster members at a single chemical composition and a single age .
Here we report new spectroscopic observations of the G type , detached eclipsing binary EPIC 219394517 in the open cluster Ruprecht 147 ( { [ Fe / H ] } = +0.10 ) , which was observed in late 2015 by the K2 mission .
A joint analysis of our radial-velocity measurements and the K2 light curve shows the 6.5 day orbit to be very nearly circular .
We derive highly precise masses of 1.0782 ^ { +0.0019 } _ { -0.0019 } \mathcal { M } _ { \sun } ^ { N } and 1.0661 ^ { +0.0027 } _ { -0.0021 } \mathcal { M } _ { \sun } ^ { N } , radii of 1.055 \pm 0.011 \mathcal { R } _ { \sun } ^ { N } and 1.042 \pm 0.012 \mathcal { R } _ { \sun } ^ { N } , and effective temperatures of 5930 \pm 100 K and 5880 \pm 100 K for the primary and secondary , respectively .
The distance we infer , 283 ^ { +18 } _ { -16 } pc , corresponds to a parallax in good agreement with the Gaia /DR2 value for the star .
Current stellar evolution models from the MIST and PARSEC series match the above physical properties very well at ages of 2.48 and 2.65 Gyr .
Isochrones for these same ages and the measured composition , along with our reddening estimate for EPIC 219394517 , also show generally good agreement with the optical and near-infrared color-magnitude diagrams of the cluster , which can be constructed with no free parameters as the distances of all member stars are known from Gaia .