Abstract
Variability of optical polarized light has result to be a powerful tool for studying Blazars, since it enable us to determine the strength and orientation of magnetic field associated with the relativistic jet. Also, polarized light is a valuable way of testing the location of the γ-ray emission, although the physical mechanisms responsible of its production remain unclear. Most importantly, since multi-wavelength campaigns typically concentrate on sources during highly active states, the characteristic polarimetric properties of Blazars in the pre-and post outbursts states are less known. Thus, we have started a dedicated monitoring program of Blazars from the GASP sample at San Pedro Mártir, in order to study their polarimetric variability properties. In this work we want to show the current status of the program presenting some preliminary results on the optical-polarimetric and γ-ray variability observed with Fermi-LAT on the blazar PKS 1510-089.