Magnification map (see Appendix A) showing the effect of microlensing on the degree of polarization p (given in color) and on the polarization angle φ (arrows). The level of polarization is given in percent. Arrows paralell to the Y-axis correspond to φ = 0, and the arrow length corresponds to the p value. The solid black line represents the path of the source, starting from left to right. This map is a subregion of a much larger magnification map shown in Appendix A.