The mass profile of early-type galaxies in overdense environments: the case of the double source-plane gravitational lens SL2SJ02176-0513 - Tu, H; Gavazzi, R; Limousin, M; Cabanac, R; Marshall, P; Fort, B; Treu, T; Péllo, R; Jullo, E; Kneib, J; Sygnet, J (2009)