  We analyze extensive BVR_cI_c time-series photometry and radial-velocity
measurements for WOCS 40007 (Auner 259; KIC 5113053), a double-lined detached
eclipsing binary and a member of the open cluster NGC 6819. Utilizing
photometric observations from the 1-meter telescope at Mount Laguna Observatory
and spectra from the WIYN 3.5-meter telescope, we measure precise and accurate
masses (~1.6% uncertainty) and radii (~0.5%) for the binary components. In
addition, we discover a third star orbiting the binary with a period greater
than 3000 days using radial velocities and Kepler eclipse timings.
  Because the stars in the eclipsing binary are near the cluster turnoff, they
are 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 CMD
isochrone fitting and 3.1+/-0.4 Gyr by analyzing the mass-radius (M-R) data for
this binary. The M-R age is above previous determinations for this cluster, but
consistent within 1 sigma uncertainties. When the M-R data for the primary star
of the additional cluster binary WOCS 23009 is included, the weighted age
estimate drops to 2.5+/-0.2 Gyr, with a systematic uncertainty of at least 0.2
Gyr. The age difference between our CMD and M-R findings may be the result of
systematic error in the metallicity or helium abundance used in models, or due
to slight radius inflation of one or both stars in the WOCS 40007 binary.
