Effective magnetic field and its uncertainty in gauss versus rotational phase. Note that the field is extraordinarily weak and the phase of its maximum coincides exactly with the maximum of brightness. Measurements taken form: ◦ – Borra & Lanstreet (1980); ◇ – Bohlender & Landstreet (1990);  △  – Donati & Semel (1990); □ – Wade et al. (2000). Areas of individual markers correspond to the weight of the particular measurement, which is inversely proportional to the squares of their uncertainties.