  Postulate of SL(2,\Z) invariance of toroidally compactified heterotic string
theory in four dimensions implies the existence of new string (dual string)
states carrying both, electric and magnetic charges. In this paper we study the
interaction between these dual strings. In particular, we consider scattering
of two such strings in the limit where one string passes through the other
string without touching it, and show that such a scattering leads to the
exchange of a fixed amount of electric and magnetic charges between the two
strings.
