Uniqueness of photon sphere for Einstein-Maxwell-dilaton black holes   with arbitrary coupling constant
  The uniqueness of static asymptotically flat photon sphere for static black hole solution in Einstein-Maxwell-dilaton theory with arbitrary coupling constant was proposed. Using the conformal positive energy theorem we show that the dilaton sphere subject to the non-extremality condition authorizes a cylinder over a topological sphere. 
Marek Rogatko
hep-th gr-qc