Polarization and photometric observations of the gamma-ray blazar PG 1553+113 I. Andruchow, J. A. Combi, A. J. Muñoz-Arjonilla, G. E. Romero, S. A. Cellone and J. Martí A&A, 531 (2011) A38 Published online: 07 June 2011 DOI: 10.1051/0004-6361/201016009