Generalized $\mu$-Terms from Orbifolds and M-Theory
  We consider solutions to the $\mu$-problem originating in the effective low energy theories, of N=1 orbifold compactifications of the heterotic string, after supersymmetry breaking. They are consistent with the invariance of the one loop corrected effective action in the linear representation of the dilaton. The proposed $\mu$-terms naturally generalize solutions proposed previously, in the literature, in the context of minimal low energy supergravity models. They emanate from the connection of the non-perturbative superpotential to the determinant of the mass matrix of the chiral compactification modes. Within this approach we discuss the lifting of our solutions to their M-theory compactification counterparts. 
Christos Kokorelis