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 ± 0.00001 days. We find masses 3.604 −0.044 R 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 ± 15 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.

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