STABLE NON-BPS BOUND-STATES OF BPS D-BRANES

S-duality symmetry of type IIB string theory predicts the existence of a stable non-BPS state on an orbifold five plane of the type IIB theo ry if the orbifold group is generated by the simultaneous action of (- 1)(FL) and the reversal of sign of the four coordinates transverse to the orbifold plane. We calculate the mass of this state by starting fr om a pair of D-strings carrying the same charge as this state, and the n identifying the point in the moduli space where this pair develops a tachyonic mode, signalling the appearance of a bound state of this co nfiguration into the non-BPS state.