Coordinated multisite observations of the variability of BL Lac

The authors present the results of multisite observations of the optical polarization of BL Lac, together with some flux measurements, at a time when BL Lac was relatively faint. The level of polarization varies in a manner similar to that reported by Moore et al., although the position angle behaviour is quite different, varying only a small amount from a mean of 28°. Models are presented to account for the observed polarization behaviour of BL Lac. The best model is one consisting of two polarized components, one having a variable polarization, intensity and random position angle, and the other a fixed polarization, intensity and position angle. The authors tentatively identify this latter component as being associated with the inner region of the optical jet of BL Lac.