Surface abundances of ON stars F. Martins, S. Simón-Díaz, A. Palacios, I. Howarth, C. Georgy, N. R. Walborn, J.-C. Bouret and R. Barbá A&A, 578 (2015) A109 Published online: 12 June 2015 DOI: 10.1051/0004-6361/201526130