We present time-series radial-velocity and photometric observations of a solar-type double-lined eclipsing binary star ( V 12 ) in the old open cluster NGC 188 .
We use these data to determine the spectroscopic orbit and the photometric elements for V 12 .
From our analysis we determine accurate masses ( M _ { p } = 1.103 \pm 0.007 ~ { } M _ { \odot } , M _ { s } = 1.081 \pm 0.007 ~ { } M _ { \odot } ) and radii ( R _ { p } = 1.424 \pm 0.019 ~ { } R _ { \odot } , R _ { s } = 1.373 \pm 0.019 ~ { } R _ { \odot } ) for the primary ( p ) and secondary ( s ) binary components .
We adopt a reddening of E _ { B - V } = 0.087 for NGC 188 , and derive component effective temperatures of 5900 \pm 100 K and 5875 \pm 100 K , respectively , for the primary and secondary stars .
From their absolute dimensions , the two components of V 12 yield identical distance moduli of V _ { 0 } - M _ { V } = 11 \fm 24 \pm 0 \fm 09 , corresponding to 1770 \pm 75 pc .
Both stars are near the end of their main-sequence evolutionary phase , and are located at the cluster turnoff in the color-magnitude diagram .
We determine an age of 6.2 \pm 0.2 Gyr for V 12 and NGC 188 , from a comparison with theoretical isochrones in the mass-radius diagram .
This age is independent of distance , reddening , and color-temperature transformations .
We use isochrones from Victoria-Regina and Yonsei-Yale with [ Fe/H ] = -0.1 and [ Fe/H ] = 0.0 .
From the solar metallicity isochrones , an age of 6.4 Gyr provides the best fit to the binary components for both sets of models .
For the isochrones with [ Fe/H ] = -0.1 , ages of 6.0 Gyr and 5.9 Gyr provide the best fits for the Victoria-Regina and Yonsei-Yale models , respectively .
We use the distance and age estimates for V 12 , together with best estimates for the metallicity and reddening of NGC 188 , to investigate the locations of the corresponding VRSS and Y ^ { 2 } isochrones relative to cluster members in the color-magnitude diagram .
Plausible changes in model metallicity and distance to better match the isochrones to the cluster sequences , result in a range of ages for NGC 188 that is more than 3 times that resulting from our analysis of V 12 .