The Na-O anticorrelation in horizontal branch stars - I. NGC 2808 R. G. Gratton, S. Lucatello, E. Carretta, A. Bragaglia, V. D’Orazi and Y. Al Momany A&A, 534 (2011) A123 Published online: 19 October 2011 DOI: 10.1051/0004-6361/201117690