Open string amplitudes and large order behavior in topological string theory

We propose a formalism inspired by matrix models to compute open and closed topological string amplitudes in the B-model on toric Calabi--Yau manifolds. We find closed expressions for various open string amplitudes beyond the disk, and in particular we write down the annulus amplitude in terms of theta functions on a Riemann surface. We test these ideas on local curves and local surfaces, providing in this way generating functionals for open Gromov--Witten invariants in the spirit of mirror symmetry. In the case of local curves, we study the open string sector near the critical point which leads to 2d gravity, and we show that toric D--branes become FZZT branes in a double--scaling limit. We use this connection to determine the instanton effects that control the large order behavior of topological string theory on these backgrounds.