The α-element abundances in the most oxygen-poor planetary nebula PNG 135.9+55.9 M. G. Richer, G. Tovmassian, G. Stasińska, R. F. Jameson, P. D. Dobbie, C. Veillet, C. Gutierrez and F. Prada A&A, 395 3 (2002) 929-941 Published online: 18 November 2002 DOI: 10.1051/0004-6361:20021315