Solar calibration and the ages of the old disk clusters M67, NGC 188, and NGC 6791

The ages of the old disk star clusters M67, NGC 188, and NGC 6791 are derived on the basis of theoretical solar calibrated isochrones. The ages of M67 and NGC 188 were found to be about 4.0 +1.0 or -0.5 Gyr (close to the age of the sun) and 6.5 +1.5 or -0.5 Gyr, respectively. The age of NGC 6791 was found to be about 1.0 Gyr larger than that of NGC 188, assuming that the two clusters have the same metallicity. If, however, NGC 6791 is more metal rich, its age is less certain; it could be as low as 6.5 Gyr.