An M-Theory Perspective on Heterotic K3 Orbifold Compactifications
  We analyze the structure of heterotic M-theory on K3 orbifolds by presenting a comprehensive sequence of M-theoretic models constructed on the basis of local anomaly cancellation. This is facilitated by extending the technology developed in our previous papers to allow one to determine "twisted" sector states in non-prime orbifolds. These methods should naturally generalize to four-dimensional models, which are of potential phenomenological interest. 
M. Faux, D. Lust and B.A. Ovrut