D-brane instantons on non-Spin cycles
  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. 
Per Berglund and I\~naki Garc\'ia-Etxebarria
hep-th