  Models of rotating single stars can successfully account for a wide variety
of observed stellar phenomena, such as the surface enhancements of N and He.
However, recent observations have questioned the idea that rotational mixing is
the main process responsible for the surface enhancements, emphasizing the need
for a strong and conclusive test.
  We investigate the consequences of rotational mixing for massive
main-sequence stars in short-period binaries. In these systems the tides spin
up the stars to rapid rotation. We use a state-of-the-art stellar evolution
code including the effect of rotational mixing, tides, and magnetic fields. We
discuss the surface abundances expected in massive close binaries (M1~20 solar
masses) and we propose using such systems to test the concept of rotational
mixing. As these short-period binaries often show eclipses, their parameters
can be determined with high accuracy, allowing for a direct comparison with
binary evolution models.
  In more massive close systems (M1~50 solar masses, Porb<~2 days) we find that
helium is efficiently mixed throughout the envelope. The star remains blue and
compact during the main-sequence phase. It stays within its Roche lobe while it
gradually becomes a helium star. It is the less massive star, in which the
effects of rotational mixing are less pronounced, which fills its Roche lobe
first. We propose that this evolution path provides an alternative channel for
the formation of tight Wolf-Rayet binaries with a main-sequence companion and
might explain massive black hole binaries such as the intriguing system M33
X-7.
