  The Higgs mixing term coefficient $\mu_{eff}$ is calculated in the scalar
potential in supergravity theories with string origin, in a model independent
approach. A general low energy effective expression is derived, where new
contributions are included which depend on the modular weights $q_{1,2}$ of the
Higgs superfields, the moduli and derivative terms. We find that in a class of
models obtained in the case of compactifications of the heterotic superstring,
the derivative terms are identically zero. Further, the total $\mu_{eff}$-term
vanishes identically if the sum of the two modular weights $q_1+q_2$ is equal
to two. Subleading $\mu$- corrections, in the presence of intermediate gauge
symmetries predicted in viable string scenarios, are also discussed.
