Line ratios for helium-like ions: Applications to collision-dominated plasmas D. Porquet, R. Mewe, J. Dubau, A. J. J. Raassen and J. S. Kaastra A&A, 376 3 (2001) 1113-1122 Published online: 15 September 2001 DOI: 10.1051/0004-6361:20010959