The 3-D ionization structure of NGC 6818: A Planetary Nebula threatened by recombination S. Benetti, E. Cappellaro, R. Ragazzoni, F. Sabbadin and M. Turatto A&A, 400 1 (2003) 161-183 Published online: 24 February 2003 DOI: 10.1051/0004-6361:20021690