  We study the structure of protoneutron stars within the finite temperature
Brueckner-Bethe-Goldstone many-body theory. If nucleons, hyperons, and leptons
are present in the stellar core, we find that neutrino trapping stiffens
considerably the equation of state, because hyperon onsets are shifted to
larger baryon density. However, the value of the critical mass turns out to be
smaller than the ``canonical'' value 1.44 $M_\odot$. We find that the inclusion
of a hadron-quark phase transition increases the critical mass and stabilizes
it at about 1.5--1.6 $M_\odot$.
