Exploring the SO(32) Heterotic String
  We give a complete classification of Z_N orbifold compactification of the heterotic SO(32) string theory and show its potential for realistic model building. The appearance of spinor representations of SO(2n) groups is analyzed in detail. We conclude that the heterotic SO(32) string constitutes an interesting part of the string landscape both in view of model constructions and the question of heterotic-type I duality. 
Hans Peter Nilles, Saul Ramos-Sanchez, Patrick K.S. Vaudrevange, Akin   Wingerter