OPERATOR PRODUCT EXPANSION OF THE ENERGY-MOMENTUM TENSOR IN 2D CONFORMAL FIELD-THEORIES ON MANIFOLDS WITH BOUNDARY

Starting from the well-known expression for the trace anomaly we deriv e the T T operator product expansion of the energy-momentum tenser in 2D conformal theories defined in the upper halfplane without making us e of the additional condition of no energy-momentum flux across the bo undary. The OPE turns out to be the same as in the absence of the boun dary. For this result it is crucial that the trace anomaly is proporti onal to the Gauss-Bonnet density. Some relations to the sigma-model ap proach for open strings are discussed.