Notes on unoriented D-brane instantons

We review basic aspects of worldsheet and penta-brane instantons as well as (unoriented) D-brane instantons, which is our main focus here, and threshold corrections to BPS-saturated couplings. We then consider nonperturbative superpotentials generated by "gauge" and "exotic" instantons living on D3-branes at orientifold singularities. Moreover, we discuss the interplay between worldsheet and D-string instantons on T-4/Z(2). We focus on a 4-Fermi amplitude, give Heterotic and perturbative Type I descriptions, and offer a multi-D-string instanton interpretation. We conclude with possible interesting developments.