Z_2 x Z_2 Heterotic Orbifold Models of Non Factorisable Six Dimensional   Toroidal Manifolds
  We discuss heterotic strings on Z_2 x Z_2 orbifolds of non factorisable six-tori. Although the number of fixed tori is reduced as compared to the factorisable case, Wilson lines are still needed for the construction of three generation models. An essential new feature is the straightforward appearance of three generation models with one generation per twisted sector. We illustrate our general arguments for the occurrence of that property by an explicit example. Our findings give further support for the conjecture that four dimensional heterotic strings formulated at the free fermionic point are related to Z_2 x Z_2 orbifolds. 
Alon E. Faraggi, Stefan Forste, Cristina Timirgaziu