  Rotational mixing is a very important but uncertain process in the evolution
of massive stars. We propose to use close binaries to test its efficiency.
Based on rotating single stellar models we predict nitrogen surface
enhancements for tidally locked binaries. Furthermore we demonstrate the
possibility of a new evolutionary scenario for very massive (M > 40 solar mass)
close (P < 3 days) binaries: Case M, in which mixing is so efficient that the
stars evolve quasi-chemically homogeneously, stay compact and avoid any
Roche-lobe overflow, leading to very close (double) WR binaries.
