A Comment on BPS States in F-theory in 8 Dimensions
  We study some aspects of enhanced gauge symmetries in F-theory compactified on K3. We find open string configurations connecting various 7-branes which represent stable BPS states. In this approach we recover $D_n$ and $E_n$ gauge groups previously found from an analysis of sigularities of the moduli space of elliptically fibered K3 manifolds as well as examples of non-perturbative realizations of $A_n$ groups. 
A. Johansen