Analysis of the long-term polarization behaviour of BL Lac V. A. Hagen-Thorn, E. G. Larionova, S. G. Jorstad, C.-I. Björnsson and V. M. Larionov A&A, 385 1 (2002) 55-61 Published online: 15 April 2002 DOI: 10.1051/0004-6361:20020145