Extending the Black Hole Uniqueness Theorems, I. Accelerating Black   Holes: The Ernst Solution and C-Metric
  We present black hole uniqueness theorems for the C-metric and Ernst solution. The proof follows a similar strategy as that used to prove the uniqueness of the Kerr-Newman solution, however the presence of an acceleration horizon provides some critical differences. We also show how to derive the Bunting/Mazur result (on the positivity of a suitable divergence required in the proof) using new methods. We briefly explain the importance of the uniqueness of the Ernst solution in relation to the proposed black hole monopole pair creation mediated by the related instanton. 
Clive G. Wells