Upper five panels: Polarisation-degree time series of the polarised standards, BD +64 106 (P_s1), HD 155197 (P_s2), HD 251204 (P_s3), HILT 960 (P_s4), and VI Cyg 12 (P_s5). The dotted black lines indicate the catalogue polarisation, and the shaded regions represent the standard deviation around the mean. Bottom panels: Instrumental offset of the polarisation angle (θ_inst). The shaded regions represent the standard deviation around the mean (i.e. the solid-coloured line). Note that panel (a) contains data from the MOP-L filter, and panel (b) contains data from the MOP-I filter. The coloured-marker data do not include ellipticity corrections, whereas the black-marker data have all instrumental corrections implemented.