  We present 1D numerical simulations aimed at studying the hot-flasher
scenario for the formation of He-rich subdwarf stars. Sequences were calculated
for a wide range of metallicities and physical assumptions, such as the stellar
mass at the moment of the helium core flash. This allows us to study the two
previously proposed flavors of the hot-flasher scenario ("deep" and "shallow"
mixing cases) and to identify a third transition type. Our sequences are
calculated by solving simultaneously the mixing and burning equations within a
diffusive convection picture, and in the context of standard mixing length
theory. We are able to follow chemical evolution during deep-mixing events in
which hydrogen is burned violently, and therefore able to present a homogeneous
set of abundances for different metallicities and varieties of hot-flashers. We
extend the scope of our work by analyzing the effects of non-standard
assumptions, such as the effect of chemical gradients, extra-mixing at
convective boundaries, possible reduction in convective velocities, or the
interplay between difussion and mass loss. Particular emphasis is placed on the
predicted surface properties of the models.
  We find that the hot-flasher scenario is a viable explanation for the
formation and surface properties of He-sdO stars. Our results also show that,
during the early He-core burning stage, element diffusion may produce the
transformation of (post hot-flasher) He-rich atmospheres into He-deficient
ones. If this is so, then we find that He-sdO stars should be the progenitors
of some of the hottest sdB stars.
