MODULAR SYMMETRIES, THRESHOLD CORRECTIONS AND MODULI FOR Z(2)XZ(2) ORBIFOLDS

Z(2) x Z(2) Coxeter orbifolds are constructed with the property that s ome twisted sectors have fixed planes for which the six-torus cannot b e decomposed into a direct sum T-2 circle plus T-4 with the fixed plan e lying in T-2. The string loop threshold corrections to the gauge cou pling constants are derived, and display symmetry groups for the T and U moduli that are subgroups of the full modular group PSL(2, Z). The effective potential for duality invariant gaugino condensate in the pr esence of hidden sector matter is constructed and minimized for the va lues of the moduli. The effect of Wilson lines on the modular symmetri es is also studied.