String Webs and Curves of Marginal Stability in Five-Dimensional E_N theories on S¹

We study curves of marginal stability (CMS) in five-dimensional N=1 E_N theories compactified on a circle using the D3-brane probe realization. In this realization, BPS states correspond to string webs in the affine E_N 7-brane background and junction positions of the webs deteremine CMS. We find that there exist string webs involving infinitely many junctions. Consequently the E_N theories have infinitely many CMS. We also find that there exists a transition from open strings to string webs involving loops. The transition describes a new phenomenon occurring on CMS.