We study the gauge and gravitational interactions of the stable non-BPS D-particles of the type I string theory. The gravitational interactions are obtained using the boundary state formalism while the SO(32) gauge interactions are determined by evaluating disk diagrams with suitable insertions of boundary changing (or twist) operators. In particular the gauge coupling of a D-particle is obtained from a disk with two boundary components produced by the insertion of two twist operators. We also compare our results with the amplitudes among the non-BPS states of the heterotic string which are dual to the D-particles. After taking into account the known duality and renormalization effects, we find perfect agreement, thus confirming at a non-BPS level the expectations based on the heterotic/type I duality.
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