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 m(F606W) similar or equal to 28.5 The white dwarf luminosity function shows a well-defined peak at m(F606W) similar or equal to 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 similar to 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 is m(F606W) similar or equal to 27.4 even more enigmatic. We discuss possible causes, none of them very convincing.