Uniqueness and nonuniqueness of the stationary black holes in 5D   Einstein-Maxwell and Einstein-Maxwell-dilaton gravity
  In the present paper we investigate the general problem of uniqueness of the stationary black solutions in 5D Einstein-Maxwell-dilaton gravity with arbitrary dilaton coupling parameter containing the Einstein-Maxwell gravity as a particular case. We formulate and prove uniqueness theorems classifying the stationary black hole solutions in terms of their interval structure, electric and magnetic charges and the magnetic fluxes. The proofs are based on the nonpositivity of the Riemann curvature operator on the space of the potentials which imposes restrictions on the dilaton coupling parameter. 
Stoytcho S. Yazadjiev
hep-th gr-qc