So-called `non-factorisable' toroidal orbifolds can be rewritten in a
factorised form as a product of three two-tori by imposing an additional shift
symmetry. This finding of Blaszczyk et al., arXiv:1111.5852, provides a new
avenue to Conformal Field Theory methods, by which the vector-like massless
matter spectrum - and thereby the type of gauge group enhancement on
orientifold invariant fractional D6-branes - and the one-loop corrections to
the gauge couplings in Type IIA orientifold theories can be computed in
addition to the well-established chiral matter spectrum derived from
topological intersection numbers among three-cycles. We demonstrate this
framework for the Z4×ΩR orientifolds on the
A3×A1×B2-type torus. As observed before for factorisable
backgrounds, also here the one-loop correction can drive the gauge groups to
stronger coupling as demonstrated by means of a four-generation Pati-Salam
example.Comment: 65 pages, 10 figures; v3: matches published version (ref +
explanatory remarks added