  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.
