Linear polarization of the leading side of Iapetus versus wavelength, observed with FORS2 at the ESO VLT during the night 2009 May 17 at a phase angle of 5.84°. In the and panels, the red lines represent measurements obtained with respect to the reference direction perpendicular to the scattering plane. The null profiles, N_U (offset to  + 0.25 % for display purposes) and N_Q, are displayed in blue, superposed on the statistical error bars (in red). The right panels show the fraction of linear polarization and its position angle, together with their error bars. The stokes is well centered on zero, which is equivalent to the position angle of polarization centered on 90.0°. In all panels, the black dotted lines show the total flux (in arbitrary units).