E6 Yukawa couplings as D-instanton effects

One of the main motivations for using F-theory as a model building tool is the fact that it can naturally generate a 10 10 5 Yukawa coupling, by engineering a so-called E6-singularity. The very fact that an exceptional group is envolved is what seemingly precludes us from understanding this in perturbative string theory.
In this talk, I will present a recent paper with I. García-Etxebarria, where we show that this Yukawa coupling can be derived as a fractional D(-1)-instanton effect at a conifold singularity. The calculation is model-independent, and requires very mild restrictions on the setup.