UBVI CCD photometry of the old open cluster Berkeley 17

Photometric UBVI CCD photometry is presented for NGC 188 and Berkeley 17. Color- magnitude diagrams ( CMDs) are constructed and reach well past the main- sequence turnoff for both clusters. Cluster ages are determined by means of isochrone fitting to the cluster CMDs. These fits are constrained to agree with spectroscopic metallicity and reddening estimates. Cluster ages are determined to be 7. 0 +/- 0. 5 Gyr for NGC 188 and 10. 0 +/- 1. 0 Gyr for Berkeley 17, where the errors refer to uncertainties in the relative age determinations. These ages are compared to the ages of relatively metal- rich inner halo/ thick- disk globular clusters and other old open clusters. Berkeley 17 and NGC 6791 are the oldest open clusters, with ages of 10 Gyr. They are 2 Gyr younger than the thick- disk globular clusters. These results confirm the status of Berkeley 17 as one of the oldest known open clusters in the Milky Way, and its age provides a lower limit to the age of the Galactic disk.