  (shortened) The peculiar chemistry observed in the multiple populations of
Galactic globular clusters is not generally found in other systems like dwarf
galaxies and open clusters, and at the moment no model can fully explain its
presence. Exploring the boundaries of the multiple population phenomenon and
the variation of its extent in the space of cluster mass, age, metallicity, and
compactness has proven to be a fruitful line of investigation. In the framework
of a larger project to search for multiple population in open clusters, based
on literature and survey data, I found peculiar chemical abundance patterns in
a sample of intermediate age open clusters with publicly available data. More
specifically, fast rotating dwarf stars (v sini > 50km/s) belonging to four
clusters display a bimodality in either [Na/Fe], [O/Fe] or both. Additionally,
two clusters show a Na-O anticorrelation in the fast rotating stars and one
cluster shows a large [Mg/Fe] variation among the stars with high [Na/Fe],
reaching the extreme Mg depletion observed in NGC 2808. Even considering that
the sample sizes are small, these patterns call for attention in the light of a
possible connection with the multiple population phenomenon of globular
clusters. The specific chemistry observed in these fast rotating dwarf stars is
thought to be produced by a complex interplay of different diffusion and mixing
mechanisms, like rotational mixing and mass loss, in turn influenced by
metallicity, binarity, mass, age, variability, and so on. However, with the
sample in hand, it was not possible to identify which stellar parameters cause
the observed Na and O bimodality and Na-O anticorrelation, suggesting that
other stellar properties might be important besides stellar rotation.
