The observations of HD 205805 obtained with FORS1 on 2005-06-12. The top panel shows the observed flux F (black solid line, in arbitrary units, and not corrected for the instrument response), the P_V = V/I profile (red solid line centred about 0), and the null profile N_V (blue solid line, offset by  − 0.75% for display purpose). The null profile is expected to be centred about zero and scattered according to a Gaussian with σ given by the P_V error bars. A typical P_V error bar is shown in the upper right of the upper panel. The slope of the interpolating lines in the bottom panels give the mean longitudinal field from P_V (left bottom panel) and from the null profile (right bottom panel), both calculated using the H Balmer and metal lines. The corresponding  ⟨ B_z ⟩  and  ⟨ N_z ⟩  values are  − 139 ± 120 G and  − 220 ± 140 G, respectively.