The influence of the C+N+O abundances on the determination of the
  relative ages of Globular Clusters: the case of NGC 1851 and NGC 6121 (M4)
  The color magnitude diagram (CMD) of NGC 1851 presents two subgiant branches
(SGB), probably due the presence of two populations differing in total CNO
content. We test the idea that a difference in total CNO may simulate an age
difference when comparing the CMD of clusters to derive relative ages. We
compare NGC 1851 with NGC 6121 (M4), a cluster of very similar [Fe/H]. We find
that, with a suitable shift of the CMDs that brings the two red horizontal
branches at the same magnitude level, the unevolved main sequence and red giant
branch match, but the SGB of NGC 6121 and its red giant branch "bump" are
fainter than in NGC 1851. In particular, the SGB of NGC 6121 is even slightly
fainter than the the faint SGB in NGC 1851. Both these features can be
explained if the total CNO in NGC 6121 is larger than that in NGC 1851, even if
the two clusters are coeval. We conclude by warning that different initial
C+N+O abundances between two clusters, otherwise similar in metallicity and
age, may lead to differences in the turnoff morphology that can be easily
attributed to an age difference.
