Anisotropic Solutions For Orbifold Moduli From Duality Invariant Gaugino Condensates

The values of the $T$ and $U$ moduli are studied for those ${\bf Z}_N$ Coxeter orbifolds with the property that some of the twisted sectors have fixed planes for which the six-torus ${\bf T}^6$ can not be decomposed into a direct sum ${\bf T}^2\bigoplus {\bf T}^4$ with the fixed plane lying in ${\bf T}^2$. Such moduli in general transform under a subgroup of the modular group $SL(2,Z).$ The moduli are determined by minimizing the effective potential derived from a duality invariant gaugino condensate.