The nature and origin of Seyfert warm absorbers A. J. Blustin, M. J. Page, S. V. Fuerst, G. Branduardi-Raymont and C. E. Ashton A&A, 431 1 (2005) 111-125 Published online: 02 February 2005 DOI: 10.1051/0004-6361:20041775