  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.
