Rotating Dyonic Black Holes in Heterotic String Theory
  We study a class of rotating dyonic black holes in the heterotic string theory in four dimension which have left, right independent electric charges but have same magnitude for the left and right magnetic charges. In both left and right sector the electric and the magnetic vectors are orthogonal to each other. The gyromagnetic(electric) ratios are in general found not to have an upper bound. 
Dileep P. Jatkar, Sudipta Mukherji and Sudhakar Panda