  Rotation periods inferred from the magnetic variability of some Ap stars are
incredibly long, exceeding ten years in some cases. An explanation for such
slow rotation is lacking. This paper attempts to provide an explanation of the
super-slow rotation of the magnetic and thermal patterns of Ap stars in terms
of the longitudinal drift of the unstable disturbances of the kink-type
(Tayler) instability of their internal magnetic field. The rates of drift and
growth were computed for eigenmodes of Tayler instability using stellar
parameters estimated from a structure model of an A star. The computations
refer to the toroidal background magnetic field of varied strength. The
non-axisymmetric unstable disturbances drift in a counter-rotational direction
in the co-rotating reference frame. The drift rate increases with the strength
of the background field. For a field strength exceeding the (equipartition)
value of equal Alfven and rotational velocities, the drift rate approaches the
proper rotation rate of a star. The eigenmodes in an inertial frame show very
slow rotation in this case. Patterns of magnetic and thermal disturbances of
the slowly rotating eigenmodes are also computed. The counter-rotational drift
of Tayler instability eigenmodes is a possible explanation for the observed
phenomenon of super-slowly rotating Ap stars.
