XMM-Newton discovery of a Compton-thick AGN in the GPS galaxy Mkn 668 M. Guainazzi, A. Siemiginowska, P. Rodriguez-Pascual and C. Stanghellini A&A, 421 2 (2004) 461-471 Published online: 22 June 2004 DOI: 10.1051/0004-6361:20047051