The Na-O anticorrelation in horizontal branch stars - II. NGC 1851 R. G. Gratton, S. Lucatello, E. Carretta, A. Bragaglia, V. D’Orazi, Y. Al Momany, A. Sollima, M. Salaris and S. Cassisi A&A, 539 (2012) A19 Published online: 20 February 2012 DOI: 10.1051/0004-6361/201118491