NUCLEATING BLACK-HOLES VIA NONORIENTABLE INSTANTONS

We extend the analysis of black hole pair creation to include nonorien table instantons. We classify these instantons in terms of their funda mental 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 therm odynamical properties of nonorientable black holes and find that in th e nonextreme case there are interesting modifications of the usual for mulas for temperature and entropy.