Chern-Simons from Dirichlet 2-brane instantons
  In the vicinity of points in Calabi-Yau moduli space where there are degenerating three-cycles the low energy effective action of type IIA string theory will contain significant contributions arising from membrane instantons that wrap around these three-cycles. We show that the world-volume description of these instantons is Chern-Simons theory. 
Martin O'Loughlin