Non-simply laced Lie algebras via F theory strings

In order to describe the appearance in F theory of the non-simply-laced Lie algebras, we use the representation of symmetry enhancements by means of string junctions. After an introduction to the techniques used to describe symmetry enhancement, that is algebraic geometry, BPS states analysis and string junctions, we concentrate on the latter. We give an explicit description of the folding of D2n to Bn, of the folding of E6 to F4 and that of D4 to G2 in terms of junctions and Jordan strings. We also discuss the case of Cn, but we are unable in this case to provide a string interpretation.