Stokes parameters observed towards Musca (left), B211(middle), and L1506 (right). The profiles correspond to the observed I (top), Q (middle) and U (bottom) emission averaged along the filament crest as explained in Sect. 4.1. The errorbars represent the dispersion of the pixel values that have been averaged at a given r. The polynomial fits to the background are also shown. The vertical dashed lines indicate the position of the outer radius R_out for each filament.