2021ApJ...921..133T

We report follow-up spectroscopic observations of the 1.62 day, K-type, detached, active, near-circular, double-lined eclipsing binary EPIC 219511354 in the open cluster Ruprecht 147, identified previously on the basis of photometric observations from the Kepler/K2 mission. This is the fourth eclipsing system analyzed in this cluster. A combined analysis of the light curve and radial velocities yields accurate masses of M_Aa = 0.912 ± 0.013 M_☉ and M_Ab = 0.822 ± 0.010 M_☉ for the primary (star Aa) and secondary (Ab), along with radii of R_Aa = 0.920 ±0.016 R_☉ and R_Ab = 0.851 ± 0.016 R_☉, and effective temperatures of 5035 ± 150 and 4690 ± 130 K, respectively. Comparison with current models of stellar evolution for the known age and metallicity of the cluster reveals that both radii are larger (by 10%-14%) and both temperatures cooler (by ∼6%) than theoretically predicted, as is often seen in M dwarfs. This is likely caused by the significant stellar activity in the system, manifested here by 6% peak-to-peak out-of-eclipse variability, a filled-in Hα line, and its detection as an X-ray source. We also find EPIC 219511354 to be a hierarchical triple system, with a low-mass tertiary in an eccentric 220 day orbit.