Fractional linear polarization vs. reddening. Open circles show the p_mean of the DPs, while filled circles show the debiased (Table 3). The E(B–V) is found from the LHD map as explained in the text. The solid black line indicates the upper envelope from Eq. (5) (Hiltner 1956). The expression for the mean p, Eq. (6) (Fosalba et al. 2002), is traced by the dashed gray line. The gray regions indicate the interval in which E(B–V) values lie within 99% confidence.