Fraction of circular polarisation of the background around star WD 0708−670 observed on 2019-10-17. The red, magenta, green and black solid lines show V/I at various distances from the star as indicated in the figure. The blue solid line represents the null profile (offset by − 4% for display purpose, and with errorbars in light blue colour) corresponding to the V/I spectrum measured at 45′′ from the star (magenta line). This profile is representative of all other null profiles.