  We present the results of a high spectral resolution study of the eclipsing
binary AR Aur. AR Aur is the only known eclipsing binary with a HgMn primary
star exactly on the ZAMS and a secondary star still contracting towards the
ZAMS. We detect for the first time in the spectra of the primary star that for
many elements the line profiles are variable over the rotation period. The
strongest profile variations are found for the elements Pt, Hg, Sr, Y, Zr, He
and Nd, while the line profiles of O, Na, Mg, Si, Ca, Ti, and Fe show only weak
distortions over the rotation period. The slight variability of He and Y is
also confirmed by the study of high resolution spectra of another HgMn star:
alpha And. A preliminary modelling of the inhomogeneous distribution has been
carried out for Sr and Y. Our analysis shows that these elements are very
likely concentrated in a fractured ring along the rotational equator. It may be
an essential clue for the explanation of the origin of the chemical anomalies
in HgMn stars (which are very frequently found in binary and multiple systems)
that one large fraction of the ring is missing exactly on the surface area
which is permanently facing the secondary, and another small one on the almost
opposite side. The results presented about the inhomogeneous distribution of
various chemical elements over the stellar surface of the primary suggest new
directions for investigations to solve the question of the origin of abundance
anomalies in B-type stars with HgMn peculiarity.
