Ap stars with variable periods

The majority of magnetic chemically peculiar (mCP) stars exhibit periodic light, magnetic, radio, and spectroscopic variations that can be adequately modelled as a rigidly-rotating main-sequence star with persistent surface structures. Nevertheless, there is a small sample of diverse mCP stars whose rotation periods vary on timescales of decades while the shapes of their phase curves remain unchanged. Alternating period increases and decreases have been suspected in the hot CP stars CU Vir and V901 Ori, while rotation in the moderately cool star BS Cir has been decelerating. These examples bring new insight into this theoretically unpredicted phenomenon. We discuss possible causes of such behaviour and propose that dynamic interactions between a thin, outer, magnetically-confined envelope braked by the stellar wind, and an inner faster-rotating stellar body are able to explain the observed rotational variability