Gauge and gravitational interactions of non-BPS D-particles

We study the gauge and gravitational interactions of the stable non-BPS D-p articles of the type I string theory. The gravitational interactions are ob tained using the boundary state formalism while the SO(32) gauge interactio ns are determined by evaluating disk diagrams with suitable insertions of b oundary 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 t he amplitudes among the non-BPS states of, the heterotic string which are d ual to the D-particles. After taking into account the known duality and ren ormalization effects, we find perfect agreement, thus confirming at a non-B PS level the expectations bused on the heterotic/type I duality. (C) 2000 E lsevier Science B.V. All rights reserved.