Abundances of planetary nebulae NGC 40 and NGC 6153 S. R. Pottasch, J. Bernard-Salas, D. A. Beintema and W. A. Feibelman A&A, 409 2 (2003) 599-609 Published online: 17 November 2003 DOI: 10.1051/0004-6361:20031122