  We present an extensive set of photometry and radial velocities for the
detached eclipsing binary KV 29 in the intermediate-aged open cluster M11 (NGC
6705). Spectroscopy shows that the system is double-lined and all available
evidence (proper motion, photometry, and position on the CMD) indicates it is a
member of the cluster. We find the period of the binary to be
$4.64276\pm0.00001$ days. We find masses $3.604^{+0.002}_{-0.011}M_\odot$ and
$1.837^{+0.001}_{-0.006}M_\odot$, and radii $5.392^{+0.018}_{-0.035}R_\odot$
and $1.656^{+0.007}_{-0.044}R_\odot$ for the primary and secondary stars,
respectively. Because the primary star in the binary is rapidly evolving and is
brighter than the cluster turnoff in a color-magnitude diagram, the measurement
of its radius leads to a strong constraint on the cluster age. We find the age
of M11 to be $222^{+2}_{-3}\pm15$ Myr, where the quoted uncertainties come from
statistical errors in the calculated masses and radii, and systematic
uncertainties due to the ambiguity of the metallicity of the open cluster and
variations within the isochrone models concerning heavy elements and convective
overshooting.
