Polarization measurements of DPN1. Left: q − u measurements of individual stars of DPN1 (blue), together with the weighted mean and the error on the weighted mean (red). The black circle marks the value p = 0.1%. Right: q (green) and u (blue) measurements of the stars in the patch as a function of the distance from Earth. The green (blue) dashed line corresponds to the weighted mean q (u) of the patch and the green (blue) shaded region to its error on the weighted mean.