  By including the effects of superstring thresholds, we reconsider minimal
string unification together with the requirement of producing a
supersymmetry-breaking gluino condensate in the hidden sector. This gives, for
examples of phenomenologically-viable $Z_8'$ and other related orbifolds, the
constraint that $0.1215 \leq \alpha_s(M_Z) \leq 0.1270$. In such models, a
hidden photino can be a source of cosmological dark matter detectable by
gravitational microlensing.
