Scattering of twist fields from D-branes and orientifolds

We compute the two-point function for Z_N orbifold twist fields on the disk and RP2. We apply this to a computation of the O(g_s) correction to the Kahler potential for (the symmetric combination of) blow-up modes in type I string theory on T^6/Z_3. This is related by supersymmetry to the one-loop field dependent correction to the Fayet-Iliopoulos D-term for the anomalous U(1) factor. We find this correction to be non-vanishing away from the orbifold point.