Upper panel: mean longitudinal magnetic field of HD 965 against rotation phase. To distinguish measurements obtained at similar phases in different cycles, circles (blue) were used to identify observations acquired until 2008; squares (brown) correspond to spectra taken since 2009. Filled symbols are used for the measurements of Mathys (2017), and open symbols for all determinations based on SAO spectra. The long-dashed line (red) is the best fit of the observations by a cosine wave, see Eq. (4). The short-dashed line (dark green) corresponds to the superposition of low-order multipoles discussed in Sect. 4. Lower panel: differences O − C between individual ⟨B_z⟩ measurements and best fit curve, against rotation phase. The dotted lines (blue) correspond to ±1 rms deviation of the observational data about the fit (red dashed line). The symbols are the same as in the upper panel.