Quasi-quantum groups from Kalb-Ramond fields and magnetic amplitudes for   strings on orbifolds
  We present the general form of the operators that lift the group action on the twisted sectors of a bosonic string on an orbifold ${\cal M}/G$, in the presence of a Kalb-Ramond field strength $H$. These operators turn out to generate the quasi-quantum group $D_{\omega}[G]$, introduced in the context of orbifold conformal field theory by R. Dijkgraaf, V. Pasquier and P. Roche. The 3-cocycle $\omega$ entering in the definition of $D_{\omega}[G]$ is related to $H$ by a series of cohomological equations in a tricomplex combining de Rham, Cech and group coboundaries. We construct magnetic amplitudes for the twisted sectors and show that $\omega=1$ arises as a consistency condition for the orbifold theory. Finally, we recover discrete torsion as an ambiguity in the lift of the group action to twisted sectors, in accordance with previous results presented by E. Sharpe. 
J.-H. Jureit, T. Krajewski