The polarised spectrum of HD 94660 observed on 2007-03-22 with grism 600B. The top panel shows the normalised derivative of the observed flux, and the panel immediately below shows the observed flux F (black solid line, in arbitrary units, and not corrected for the instrument response), the P_V profile (red solid line centred about 0), and the null profile (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. The P_V error bars are represented with light blue bars centred about –0.75%. The slope of the interpolating lines in the bottom panels gives the mean longitudinal field from P_V (left bottom panel) and from the null profile (right bottom panel) both calculated using the H Balmer lines only. The corresponding  ⟨ B_z ⟩  and  ⟨ N_z ⟩  values are  − 1885 ± 90 G and  − 192 ± 62 G, respectively.