Ingo Runkel (King's Coll.)
One way to formulate orbifold models is to allow field configurations which are not periodic along a circular path on the world sheet, but are allowed to jump according to the action of the orbifold group. An equivalent way to think about this is to embed a network of `defect lines' in the world sheet and allow the fields to jump across the defect lines. When writing down the action for WZW models in the presence of defect lines, one finds both, an obstruction and an ambiguity in the definition of the orbifold model. The same happens in the quantised WZW model.
Arnold Sommerfeld Center