Nucleating Black Holes via Non-Orientable Instantons
  We extend the analysis of black hole pair creation to include non- orientable 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 analyse the thermodynamical properties of non-orientable black holes and find that in the non-extreme case, there are interesting modifications of the usual formulae for temperature and entropy. 
A. Chamblin (Institute for Theoretical Physics, UCSB), G.W. Gibbons   (DAMTP, University of Cambridge)