  Using single star models including the effects of shellular rotation with and
without magnetic fields, we show that massive stars at solar metallicity with
initial masses lower than about 20-25 M$_\odot$ and with an initial rotation
above $\sim 350$ km s$^{-1}$ likely reach the critical velocity during their
Main-Sequence phase. This results from the efficient outwards transport of
angular momentum by the meridional circulation. This could be a scenario for
explaining the Be stars. After the Main-Sequence phase, single star in this
mass range can again reach the critical limit when they are on a blue loop
after a red supergiant phase \citep{HL98}. This might be a scenario for the
formation of B[e] stars, however as discussed by Langer & Heger (1998), this
scenario would predict a short B[e] phase (only some 10$^4$ years) with
correspondingly small amounts of mass lost.
