Charge, geometry, and effective mass in the Kerr-Newman solution to the   Einstein field equations
  It has been shown that for the Reissner-Nordstrom solution to the vacuum Einstein field equations charge, like mass, has a unique space-time signature [Found. Phys. 38, 293-300 (2008)]. The presence of charge results in a negative curvature. This work, which includes a discussion of effective mass, is extended here to the Kerr-Newman solution. 
Gerald E. Marsh