Superstring scattering from O-planes Original Research Article

We write the vertex operators of massless NS–NS and RR states of Type II superstring theory in the presence of orientifold p-planes. They include the usual vertex operators of Type II theory and their images. We then calculate the two-point functions of these vertex operators at the projective plane image level. We show that the result can be written in the Veneziano-type formulae, with the same kinematic factor that appears in the image-branes amplitudes. While the scattering amplitudes with the usual vertex operators are not gauge invariant, the above amplitudes are invariant. From the amplitude describing scattering of two NS–NS states off the O-plane, we find the low energy effective action of O-planes. The result shows a relative factor image between couplings to O-planes and to D-branes at image order.