Constructive proof of the Kerr-Newman black hole uniqueness including   the extreme case
  A new proof of the uniqueness of the Kerr-Newman black hole solutions amongst asymptotically flat, stationary and axisymmetric electro-vacuum spacetimes surrounding a connected Killing horizon is given by means of an explicit construction of the corresponding complex Ernst potentials on the axis of symmetry. This construction, which makes use of the inverse scattering method, also works in the case of a degenerate horizon. 
Reinhard Meinel
gr-qc astro-ph.HE hep-th