  Non-perturbative effects are studied for Type I strings on the Z_3 orbifold
with Chan-Paton symmetry broken by Wilson lines. Generalizing previous analyses
that have focussed on (bi)fundamentals, it is argued that (anti)symmetric
representations of the resulting gauge group play a decisive role in generating
an ADS-like superpotential in this and related cases. Non perturbative
corrections in the closed string moduli are only allowed if properly dressed
with open string fields charged under the anomalous U(1) of the orbifold. A
general discussion of instanton effects in SYM and string theories is given.
Non-perturbative superpotentials induced by both $ED5$'s and ED1's are
analyzed. The superpotential generated by closed string fluxes, viz.
Scherk-Schwarz shifts (torsion), R-R 3-form flux as well as non-geometric
fluxes is derived. Some preliminary comments on the compatibility of the two
kinds of superpotentials and the issue of moduli stabilization are presented.
