D-branes and the conifold singularity

We analyze in detail the description of type IIB theory on a Calabi-Yau three-fold near a conifold singularity in terms of intersecting D-branes. In particular, we study the singularity structure of higher derivative F-terms of the form FgW2g where W is the gravitational superfield. This singularity is expected to be due to a one-loop contribution from a charged soliton hypermultiplet becoming massless at the conifold point. In the intersecting D-brane description this soliton is described by an open string stretched between the two D-branes. After identifying the graviphoton vertex as a closed string operator we show that F-g's have the expected singularity structure in the limit of vanishing soliton mass.