Observed Stokes V LSD profiles. The shaded grey area represents the uncertainty of each LSD profile. The null polarisation profiles are shown with the dashed green lines. The vertical bars indicate velocity intervals for the calculations of the FAP and ⟨B_z⟩ for the primary (orange bars) and secondary (red bars). The spectra are offset vertically according to the orbital phase. The phases for each observation are shown on the left-hand side and are calculated from the orbital solution derived in Sect. 3.1.