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We analyze extensive BVR_cI_c time-series photometry and radial-velocitymeasurements for WOCS 40007 (Auner 259; KIC 5113053), a double-lined detachedeclipsing binary and a member of the open cluster NGC 6819. Utilizingphotometric observations from the 1-meter telescope at Mount Laguna Observatoryand spectra from the WIYN 3.5-meter telescope, we measure precise and accuratemasses (~1.6% uncertainty) and radii (~0.5%) for the binary components. Inaddition, we discover a third star orbiting the binary with a period greaterthan 3000 days using radial velocities and Kepler eclipse timings.  Because the stars in the eclipsing binary are near the cluster turnoff, theyare evolving rapidly in size and are sensitive to age. With a metallicity of[Fe/H]=+0.09+/-0.03, we find the age of NGC 6819 to be about 2.4 Gyr from CMDisochrone fitting and 3.1+/-0.4 Gyr by analyzing the mass-radius (M-R) data forthis binary. The M-R age is above previous determinations for this cluster, butconsistent within 1 sigma uncertainties. When the M-R data for the primary starof the additional cluster binary WOCS 23009 is included, the weighted ageestimate drops to 2.5+/-0.2 Gyr, with a systematic uncertainty of at least 0.2Gyr. The age difference between our CMD and M-R findings may be the result ofsystematic error in the metallicity or helium abundance used in models, or dueto slight radius inflation of one or both stars in the WOCS 40007 binary.

WOCS 40007: A Detached Eclipsing Binary near the Turnoff of the Open Cluster NGC 6819