Characterization of the optical polarimetric variability in a sample of blazars

Brightness variations across the entire electromagnetic spectrum, occurring at several time scales, are known properties of blazars. The variability of the continuum emission may be related to the formation and propagation of shock waves in the relativistic jet. In this case we may expect variations also in the degree of polarization and its position angle. In order to effectively constrain such models, we begun in 2003 a program to monitor the optical polarimetry of a sample of blazars, of which we present here the first results.