Some early-type stars with abnormal line strengths for their
intrinsic colours are investigated in order to determine their
structure and the chemical composition of their atmospheres. The
abnormal line strengths in several B-type stars (αScl, 36 Lyn,
20 Tau and HD 175156) are shown to be probably due to large ranges
of surface temperature such as would be observed in rapidly rotating
stars of normal chemical composition, if viewed pole-on. These
stars differ from the peculiar A stars, which rotate slowly and have
abnormal chemical composition. Members of the Mn group of peculiar A
stars rotate more slowly than those of the Si-4200 group, although the
mean intrinsic colours of the two groups are similar.Previous work on abundance determination in peculiar A stars
is reviewed. The chemical composition of the manganese star 53 Tau
is studied by a refined differential curve-of-growth method with
αLyr as the comparison star. Groups of lines of similar mean
excitation and ionisation potentials and wavelengths are used so
that the results are independent of the structure of 53 Tau, which
is shown to be abnormal. A new double-line peculiar A spectroscopic
binary HR 4072 was discovered, and it was found that excesses of Sr
and Y are common to both components. Spectral variations in some
peculiar A stars are described.Well established abundance abnormalities in peculiar A stars may
be explained on a modified form of van den Heuvel's theory that peculiar A
stars have been secondaries of binary systems with initial separations in
the range 10 a.u. to 100 a.u. approximately. The primaries, initially
earlier than spectral type B8, have evolved and exploded as type II
supernovae. During the explosions material enriched in heavy elements
by interior nuclear reactions was transferred to the surfaces of the
secondaries, which are now observed as peculiar A stars and are still
on the main sequence. Excesses of Be and Si and deficiencies of 0 are
caused by surface nuclear reactions on the secondaries after the
explosions of the primaries. Ln stars differ from other peculiar A
stars in many respects; the differences may be due to the primaries
expanding beyond the Roche limits before they explode. On the oblique
rotator theory, spectrum variables result from irregular distributions
of surface nuclear reactions and ion migration on the secondaries.
Some other theories of peculiar A stars are reviewed and shown to be
in conflict with observation
