Nucleating black holes via nonorientable instantons

We extend the analysis of black hole pair creation to include nonorientable instantons. We classify these instantons in terms of their fundamental symmetries and orientations. Many of these instantons admit the pin structure which corresponds to the fermions actually observed in nature, and so the natural objection that these manifolds do not admit spin structure may not be relevant. Furthermore, we analyze the thermodynamical properties of nonorientable black holes and find that in the nonextreme case there are interesting modifications of the usual formulas for temperature and entropy.