Different regimes of stability against mass transfer for MS donor stars, depending on the mass ratio and the mass of the donor. The hatched region corresponds to the critical mass ratios for which, according to Politano (1996), the mass transfer becomes dynamically unstable as a function of the donor’s mass. The dotted line indicates the shape of this regions if one uses the condition that is originally in the code (see text). The solid curve represents the critical mass ratio for zero-age MS stars of solar composition above which mass transfer occurs on the thermal time-scale. This mass ratio is derived from the thermal mass-radius exponent by using the zero-age MS mass-radius relation implemented in the code (Tout et al. 1996) and assuming that mass transfer is conservative.