The origin of the different Wolf-Rayet subtypes.

A theoretical explanation of the origin of the different Wolf-Rayet subtypes is worked out. The durations of the different evolutionary stages of massive core helium burning stars, which may be defined in terms of their surface chemical composition, have been estimated. This could be done by relating the time evolution of the surface hydrogen mass fraction to the interior hydrogen profile of stars at He-ignition via their average rates of mass loss due to stellar wind. Several observational details can be explained in this way, especially the high mean mass of WNL stars and the low mean mass of WNE stars. Further it is suggested that very massive stars explode as WNL stars.