Kaluza-Klein and H-Dyons in String Theory

Kaluza-Klein monopole and H-monopole solutions, which are T-dual to each other, are the well-known solutions of string theory compactified on $T^6$. Since string theory in this case has an S-duality symmetry, we explicitly construct the corresponding dyonic solutions by expressing the $D=4$ string effective action in a manifestly $SL(2,R)$ invariant form with an $SL(2,R)$ invariant constraint. The Schwarz-Sen charge spectrum, the BPS saturated mass formula as well as the stability of these states are discussed briefly.