Superstring scattering from O-planes

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 PR2 level. We show that the result can be written in the Veneziano-type formulae, with the same kinematic factor that appears in the Dp-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 2p-6 between couplings to O-planes and to D-branes at (α')2 order