PHOTOPOLARIMETRY OF BL LAC

This paper presents results of our monitoring of BL Lac at the 2.56 m Nordic Optical Telescope in La Palma and at the 0.91 m telescope at Na tional Astronomical Observatory in Dodaira, Japan. On La Palma we used a UBVRI-photopolarimeter which gives truly simultaneous observations in all five colour bands. At Dodaira we used a photopolarimeter which gives simultaneously photometry at B, V and R-bands and polarimetry at V-band. We have observations of BL Lac during flare behaviour and dur ing these times we have observed a clear rotation of the polarization position angle. During these flares the polarization level has also be en strongly variable and the object has lost its preferred direction o f the polarization position angle. The reason for this behaviour remai ns still unclear, but there are two possible explanations: shocks in t he helical magnetic field in the jet pointing nearly towards us or pol arized synchrotron flares rotating within the accretion disk (seen nea rly face on) of the supermassive central black hole.