The wavelength dependence of polarization in BL Lac objects

Accurate polarization measurements of six BL Lac objects and one highly polarized QSO have been obtained simultaneously at optical and near-infrared wavelengths. The polarizations observed in the infrared are generally equal to or slightly less than those in the optical. No cases are seen of very large variations in polarization or position angle with wavelength as have been previously reported for some objects. Some of the objects studied, notably AO0235 + 164, showed large night to night variations in polarization or position angle but the infrared and optical polarizations followed each other closely. The amount of polarization wavelength dependence is found to be greater when the polarization of the object is greater. For p less than 10 per cent the optical and infrared polarizations are found to be equal within the errors.