OPEN STRING - STRING JUNCTION TRANSITIONS

It is confirmed that geodesic string junctions are necessary to descri be the gauge vectors of symmetry groups that arise in the context of I IB superstrings compactified in the presence of nonlocal 7-branes. By examining the moduli space of 7-brane backgrounds for which the dilato n and axion fields are constant, we are able to describe explicitly an d geometrically how open string geodesics can fail to be smooth, and h ow geodesic string junctions then become the relevant BPS representati ves of the gauge bosons. The mechanisms that guarantee the existence a nd uniqueness of the BPS representative of any gauge vector are also s hown to generalize to the case where the dilaton and axion fields are not constant. (C) 1998 Elsevier Science B.V.