On a normal ordering ambiguity in the twisted WZW model

We clarify a normal ordering ambiguity that exists in the definition of the stress-energy tensor of the twisted WZW model, and present in detail the regularization procedure that gives rise to a modular invariant spectrum. We prove a generalization of the Goddard-Nahm-Olive theorem to the twisted case and derive the condition on the twist for which an associated heterotic string model always possesses massless spacetime fermions. (orig.)