  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.
