We study marginal stability of BPS states in four-dimensional N = 2 theories with E_N global symmetry. We use the D3-brane probe realization where BPS states correspond to string webs in the affine E_N 7-brane background. We construct string webs involving loops and find a transition between a loop string web and an open string. The transition describes a new phenomenon occurring on marginal stability curves.
展开▼