Surface brightness of Hα + [NII] sources versus orientation with respect to the radio continuum tail. An angle of 0° corresponds to a source along the direction of the stripped tail and an angle of 180° corresponds to a source directly opposite to the direction of the stripped tail. Error bars on the Hα + [NII] surface brightness are calculated following the method outlined in Sect. 2.4 and error bars on the orientation angles are determined from visual determination of tail directions (see Sect. 5.2 for details).