  The surface magnetic and abundance inhomogeneities in chemically peculiar
Ap/Bp stars are coupled and responsible for their rotationally modulated
variability. Within the framework of fossil field hypothesis these
inhomogeneities are considered to be essentially stable over the Main Sequence
(MS) timescale. However, a small group of Ap/Bp stars show rotational period
changes, which are currently not well understood. We present results of Doppler
Imaging (DI) of rapidly rotating Ap star 56 Ari for which changes in period
were previously detected. Reconstruction of the surface distribution of silicon
in 56 Ari reveals its complex spot pattern, which is responsible for the
rotationally light variability and correlated with magnetic field modulation.
Comparison of abundance maps obtained over the unprecedentedly long for such
studies interval from 1986 to 2014 confirms stability and rigid rotation of the
spot pattern. Thus, the period change in 56 Ari is not caused by rearrangement
of the surface magnetic structures and/or atomic diffusion operating on short
time scale. It is also unlikely to be explained by the visibility changes of
the spots due to free-body precession of stellar rotational axis. In the end of
the paper we briefly discuss possible alternative explanations of period
variability.
