Rotation period is plotted against the orbital period for the spectroscopic binaries for which these two periods are sufficiently constrained by the existing observations. Asterisks and crosses represent stars not known to have magnetically resolved lines, for which both periods were retrieved from the literature; crosses distinguish those for which uncertainties remain about some of the relevant parameters (see text). All other symbols correspond to Ap stars with magnetically resolved lines studied in detail in this paper. Dots identify those stars for which both periods are exactly known; open circles, stars with fully determined rotation periods for which only a lower limit of the orbital period has been obtained so far; filled triangles, stars with a known orbital period but only a lower limit of the rotation period; and open triangles, stars that have only been observed over a fraction of both their rotation and orbital periods. The short-dashed diagonal line represents the locus log P_rot = 3.9−log P_orb. The long-dashed horizontal line corresponds to P_rot = 50 d and the long-dashed vertical line to P_orb = 1000 d.