1998A&A...337..754S

A sudden lengthening of the observed rotation period of a magnetic Ap star, CU Vir, reported recently by Pyper et al. (1998, A&A, in press) is discussed in terms of the possible mechanisms which can influence the rotation period of a single star. It is shown that none of the known mechanisms is able to change the rotation period of a rigidly rotating star by the observed amount (ΔP/P ≃4.9x10^–5) so abruptly as suggested by the analysed data. If we abandon an assumption of the rigid rotation, the strength of the observed magnetic field is sufficient to modify the moment of inertia of the outer envelope of the star by the required amount. The star may be in a state of torsional oscillations resulting from an interaction of meridional circulations and internal magnetic field.