Effect of charge on negative-phase-velocity propagation of   electromagnetic waves in the ergosphere of a rotating black hole
  The presence of charge promotes the propensity of a rotating black hole to support the propagation of electromagnetic plane waves with negative phase velocity (NPV) in its ergosphere, whether the Kerr-Newman or the Kerr-Sen metric descriptions of spacetime are considered. Striking differences in the NPV characteristics for the Kerr-Newman and Kerr-Sen metrics emerge from numerical studies, particularly close to the outer event horizon when the magnitude of the charge is large. 
Benjamin M. Ross (University of Edinburgh & Pennsylvania State   University), Tom G. Mackay (University of Edinburgh), Akhle...