Observed average of the mean magnetic field modulus against rotation period. Dots: stars with known rotation periods; triangles: stars for which only the lower limit of the period is known. Open symbols are used to distinguish those stars for which existing measurements do not cover the whole rotation cycle. The horizontal and vertical dashed lines, corresponding to ⟨ B ⟩ _av = 7.5 kG and P_rot = 150 d, respectively, emphasise the absence of very strong magnetic fields in the stars with the slowest rotation (see text for details).