Towards the field theory of the Standard Model on fractional D6-branes   on T6/Z6': Yukawa couplings and masses
  We present the perturbative Yukawa couplings of the Standard Model on fractional intersecting D6-branes on T6/Z6' and discuss two mechanisms of creating mass terms for the vector-like particles in the matter spectrum, through perturbative three-point couplings and through continuous D6-brane displacements. 
Gabriele Honecker and Joris Vanhoof
hep-th