Flat Directions in $Z_{2n}$ Orbifold Models
  We study generic features related to matter contents and flat directions in $Z_{2n}$ orbifold models. It is shown that $Z_{2n}$ orbifold models have massless conjugate pairs, $R$ and $\overline R$, in certain twisted sectors as well as one of untwisted subsectors. Using these twisted sectors, $Z_{2n}$ orbifold models are classified into two types. Conjugate pairs, $R$ and $\overline R$, lead to $D$-flatness as $\langle R \rangle = \langle \overline R \rangle \neq 0$. We investigate generic superpotentials derived from orbifold models so as to show that this direction is indeed a flat direction. 
Yoshiharu Kawamura and Tatsuo Kobayashi