Upper panel: mean magnetic field modulus of HD 50169 against rotation phase. The different symbols identify the source and/or instrumental configuration from which the ⟨B⟩ value was obtained, as follows: filled circles (dark blue): CAT + CES LC; open circles (steel blue): CAT + CES SC; filled hexagon (sea green): 3.6 m + CES LC; filled triangle (violet): CFHT + Gecko (all previous from Mathys et al. 1997; Mathys 2017); five-pointed open stars (orange): 3.6 m + CES VLC; four-pointed open stars (light green): UT2 + UVES; filled pentagons (yellow): 3.6 m + HARPS; filled square (salmon): Preston (1971). The long-dashed line (blue) is the best fit of the observations by a cosine wave and its first harmonic, see Eq. (6). The short-dashed line (green) corresponds to the superposition of low-order multipoles discussed in Sect. 4. Lower panel: differences O − C between the individual ⟨B⟩ measurements and the 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.