1999AJ....118.2894S

We present high-precision UBVRI CCD photometry of the old open cluster NGC 188. Our color-magnitude diagram extends from near the red giant branch tip to as faint as ∼5 mag below the main-sequence turnoff. From an analysis of these data along with published photometry for M67, we draw the following conclusions: (1) From the UBV two-color diagram, we find a reddening of E(B-V)=0.04±0.02 for M67 and E(B-V)=0.09±0.02 for NGC 188. (2) Based on main-sequence fitting to solar abundance isochrones, the distance moduli turn out to be (m-M)_v=9.69±0.11 for M67 and (m-M)_v=11.44±0.08 for NGC 188. (3) The comparison of the CMDs to theoretical isochrones indicates that an amount of core convective overshoot equivalent to 0.10 of a pressure scale height is appropriate for M67, while no overshoot is required to fit the CMD of NGC 188. These isochrones suggest that NGC 188 is 3.0±0.7 Gyr older than M67. (4) There is a clear indication of mass segregation in both M67 and NGC 188, with the most massive stars (M/M_☉>1.1) being more centrally concentrated than those that are the least massive (0.8≥M/M_☉>0.65).