Circular polarization of the bright side of Iapetus versus wavelength, obtained with FORS2 at the ESO VLT during the night 2011-03-28 at a phase angle of 0.77°. The blue dashed curve shows the Stokes profile obtained at 0.0° instrument position angle (with respect to the north celestial pole), the green dotted curve showing the measurement obtained after rotating the instrument by 90.0° on the sky, and the red solid curve represents the average of the two signals. It can be noted that a change in signal occurs in circular polarization observations that are taken after rotating the instrument by 90 degree with respect to the intial position angle on the sky.