We show that non-Spin cycles in type IIB Calabi-Yau orientifold compactifications with vanishing B field naturally support orientifold-invariant D-brane instantons. These instantons, associated to the holomorphic cotangent bundle of the non-Spin cycle, can lead to non-perturbative corrections to the superpotential.
@article{arxiv.1210.1221,
title = {D-brane instantons on non-Spin cycles},
author = {Per Berglund and Iñaki García-Etxebarria},
journal= {arXiv preprint arXiv:1210.1221},
year = {2015}
}