Context.NGC 6791 is a unique stellar system among Galactic open clusters, which is at the same time one of the oldest open clusters and the most metal rich. It is located inside the solar circle, harbors a large population of binary stars, and possibly experienced prolonged star formation. The combination of all these properties is puzzling and poses the intriguing question of its origin.
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Jílková et al. (2012) studied this question.
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