  NGC 6791 is an old, metal-rich star cluster normally considered to be a disk
open cluster. Its red giant branch is broad in color yet, to date, there is no
evidence for a metallicity spread among its stars. The turnoff region of the
main sequence is also wider than expected from broad-band photometric errors.
Analysis of the color-magnitude diagram reveals a color gradient between the
core of the cluster and its periphery; we evaluate the potential explanations
for this trend. While binarity and photometric errors appear unlikely,
reddening variations across the face of the cluster cannot be excluded. We
argue that a viable alternative explanation for this color trend is an age
spread resulting from a protracted formation time for the cluster; the stars of
the inner region of NGC 6791 appear to be older by ~1 Gyr on average than those
of the outer region.
