Multiband optical polarimetry of BL Lacertae objects with the Nordic Optical Telescope

Optical polarization of seven selected BL Lac objects in UBVRI bands was st udied with the Nordic Optical Telescope from December 10-14, 1999. Two of t hem, 3C 66A and PKS 0735+178, were monitored for 4 nights for a total integ ration time of 4.75 and 5.5 hours, respectively. Other objects (1 Jy 0138-0 97, H 0414+009, PKS 0823-223, OJ287 and BL Lac) were observed sparsely duri ng the run. Apart from PKS 0823-223 (more polarized than observed in the pa st), the sources show levels of flux and polarization consistent with resul ts at previous epochs. 3C 66A and PKS 0735+178 were intensively observed du ring December 11 and 12 and exhibited variability of polarization, both on internight and intranight time scales. Wavelength dependence of polarizatio n has been investigated, as well as circular polarization. The results are discussed within the standard model for BL Lacs.