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We present the discovery of the totally eclipsing long-period (P = 771.8 d)binary system WOCS 23009 in the old open cluster NGC 6819 that contains both anevolved star near central hydrogen exhaustion and a low-mass (0.45 Msun) star.This system was previously known to be a single-lined spectroscopic binary, butthe discovery of an eclipse near apastron using data from the Kepler spacetelescope makes it clear that the system has an inclination that is very closeto 90 degrees. Although the secondary star has not been identified in spectra,the mass of the primary star can be constrained using other eclipsing binariesin the cluster. The combination of total eclipses and a mass constraint for theprimary star allows us to determine a reliable mass for the secondary star andradii for both stars, and to constrain the cluster age. Unlike well-measuredstars of similar mass in field binaries, the low-mass secondary is notsignificantly inflated in radius compared to model predictions. The primarystar characteristics, in combination with cluster photometry and masses fromother cluster binaries, indicates a best age of 2.62+/-0.25 Gyr, althoughstellar model physics may introduce systematic uncertainties at the ~10% level.We find preliminary evidence that the asteroseismic predictions for red giantmasses in this cluster are systematically too high by as much as 8%.

A Long-Period Totally Eclipsing Binary Star at the Turnoff of the Open Cluster NGC 6819 Discovered with Kepler