  Almost all globular clusters investigated exhibit a spread in their light
element abundances, the most studied being a Na:O anticorrelation. In contrast,
open clusters show a homogeneous composition and are still regarded as Simple
Stellar Populations. The most probable reason for this difference is that
globulars had an initial mass high enough to retain primordial gas and ejecta
from the first stellar generation and thus formed a second generation with a
distinct composition, an initial mass exceeding that of open clusters. NGC 6791
is a massive open cluster, and warrants a detailed search for chemical
inhomogeneities. We collected high resolution, high S/N spectra of 21 members
covering a wide range of evolutionary status and measured their Na, O and Fe
content. We found [Fe/H]=+0.42$\pm 0.01$, in good agreement with previous
values, and no evidence for a spread. However, the Na:O distribution is
completely unprecedented. It becomes the first open cluster to show intrinsic
abundance variations that cannot be explained by mixing, and thus the first
discovered to host multiple populations. It is also the first star cluster to
exhibit two subpopulations in the Na:O diagram with one being chemically
homogeneous while the second has an intrinsic spread that follows the
anticorrelation so far displayed only by globular clusters. NGC 6791 is unique
in many aspects, displaying certain characteristics typical of open clusters,
others more reminiscent of globulars, and yet others, in particular its Na:O
behavior investigated here, that are totally unprecedented. It clearly had a
complex and fascinating history.
