The White Dwarf Cooling Sequence in NGC 6791

In the old, populous, and metal-rich open cluster NGC 6791, we have used deep Hubble Space Telescope Advanced Camera for Surveys images to track the white dwarf cooling sequence down to mF606W~=28.5. The white dwarf luminosity function shows a well-defined peak at mF606W~=27.4 and a bending to the blue in the color-magnitude diagram. If this peak corresponds to the end of the white dwarf cooling sequence, the comparison with theoretical isochrones provides a cluster age estimate of ~2.4 Gyr, in sharp contrast with the age of 8-9 Gyr inferred from the main-sequence turnoff. If the end is at fainter magnitudes, the peak at mF606W~=27.4 is even more enigmatic. We discuss possible causes, none of them very convincing.