  Recent stellar evolution computations indicate that massive stars in the
range ~ 20 - 30 Msun are located in the blue supergiant (BSG) region of the
Hertzsprung-Russell diagram at two different stages of their life: immediately
after the main sequence (MS, group 1) and during a blueward evolution after the
red supergiant phase (group 2). From the observation of the pulsationnal
properties of a subgroup of variable BSGs (alpha Cyg variables), one can deduce
that these stars belongs to group 2. It is however difficult to simultaneously
fit the observed surface abundances and gravity for these stars, and this
allows to constrain the physical processes of chemical species transport in
massive stars. We will show here that the surface abundances are extremely
sensitive to the physics of convection, particularly the location of the
intermediate convective shell that appears at the ignition of the hydrogen
shell burning after the MS. Our results show that the use of the Ledoux
criterion to determine the convective regions in the stellar models leads to a
better fit of the surface abundances for alpha Cyg variables than the
Schwarzschild one.
