  Detailed models of alpha Cen A and B based on new seismological data for
alpha Cen B by Carrier & Bourban (2003) have been computed using the Geneva
evolution code including atomic diffusion. Taking into account the numerous
observational constraints now available for the alpha Cen system, we find a
stellar model which is in good agreement with the astrometric, photometric,
spectroscopic and asteroseismic data. The global parameters of the alpha Cen
system are now firmly constrained to an age of t=6.52+-0.30 Gyr, an initial
helium mass fraction Y_i=0.275+-0.010 and an initial metallicity
(Z/X)_i=0.0434+-0.0020. Thanks to these numerous observational constraints, we
confirm that the mixing-length parameter alpha of the B component is larger
than the one of the A component, as already suggested by many authors (Noels et
al. 1991, Fernandes & Neuforge 1995 and Guenther & Demarque 2000): alpha_B is
about 8% larger than alpha_A (alpha_A=1.83+-0.10 and alpha_B=1.97+-0.10).
Moreover, we show that asteroseismic measurements enable to determine the radii
of both stars with a very high precision (errors smaller than 0.3%). The radii
deduced from seismological data are compatible with the new interferometric
results of Kervella et al. (2003) even if they are slightly larger than the
interferometric radii (differences smaller than 1%).
