Context : Stellar evolution models of massive stars are very sensitive to the adopted mass-loss scheme .
The magnitude and evolution of mass-loss rates significantly affect the main sequence evolution , and the properties of post-main sequence objects , including their rotational velocities .

Aims : Driven by potential discrepancies between theoretically predicted and observationally derived mass-loss rates in the OB star range , we aim in particular to investigate the response to mass-loss rates that are lower than currently adopted , in parallel with the mass-loss behavior at the “ first ” bi-stability jump .

Methods : We performed 1D hydrodynamical model calculations of single 20 - 60 \mathrm { M _ { \odot } } Galactic ( Z = 0.014 ) stars where the effects of stellar winds are already significant in the main sequence phase .
We have developed an experimental wind routine to examine the behavior and response of the models under the influence of different mass-loss rates .
This observationally guided , simple and flexible wind routine is not a new mass-loss description but a useful tool based on the wind-momentum luminosity relation and other scaling relations , and provides a meaningful base for various tests and comparisons .

Results : The main result of this study indicates a dichotomy between solutions of currently debated problems regarding mass-loss rates of hot massive stars .
In a fully diffusive approach , and for commonly adopted initial rotational velocities , lower mass-loss rates than theoretically predicted require to invoke an additional source of angular momentum loss ( either due to bi-stability braking , or yet unidentified ) to brake down surface rotational velocities .
On the other hand , a large jump in the mass-loss rates due to the bi-stability mechanism ( a factor of 5 - 7 predicted by Vink et al .
( 2000 , Astronomy & Astrophysics , 362 , 295 ) , but a factor of 10 - 20 in modern models of massive stars ) is challenged by observational results , and might be avoided if the early mass-loss rates agreed with the theoretically predicted values .

Conclusions : We conclude that simultaneously adopting lower mass-loss rates and a significantly smaller jump in the mass-loss rates over the bi-stability region ( both compared to presently used prescriptions ) would require an additional mechanism for angular momentum loss to be present in massive stars .
Otherwise , the observed rotational velocities of a large population of B supergiants , that are thought to be the evolutionary descendants of O stars , would remain unexplained .