  We explore the possibility that the anomalous split in the Subgiant branch of
the galactic globular cluster NGC 1851 is due to the presence of two distinct
stellar populations with very different initial metal mixtures: a normal
alpha-enhanced component, and one characterized by strong anticorrelations
among the CNONa abundances, with a total CNO abundance increased by a factor of
two. We test this hypothesis taking into account various empirical constraints,
and conclude that the two populations should be approximately coeval, with the
same initial He-content. More high-resolution spectroscopical measurements of
heavy elements -- and in particular of the CNO sum -- for this cluster are
necessary to prove (or disprove) this scenario.
