  The moduli dependence of string loop threshold corrections to gauge coupling
constants is investigated for those ${\bf Z}_N$ Coxeter orbifolds with the
property that some twisted sectors have fixed planes for which the six-torus
${\bf T}_6$ can not be decomposed into a direct sum ${\bf T}_4 \bigoplus{\bf
T}_2$ with the fixed plane lying in ${\bf T}_2.$
