MOPTOP instrumental depolarisation for the (a) MOP-L and (b) MOP-I filters. The P_ref is the polarisation degree of BD +64 106, VI Cyg 12, HD 251204, HD 155197, and HILT 960 reported in Turnshek et al. (1990), Schmidt et al. (1992), Weitenbeck (1999), and P is that measured with the MOPTOP. The dotted line represents the 1:1 relation. The solid lines are linear fits to the data, i.e. P = P_ref D, where D is the depolarisation factor. Coloured-marker data (D_c) do not include ellipticity corrections, and black-marker data (D_ϵ) are corrected for ellipticity.