Timelike Duality
  Several stationary solutions of the low energy string equations are dualized with respect to their timelike symmetry. Many of the duals have simple physical interpretations. Those of the nonextremal three dimensional black hole and black string are negative mass black strings. The extremal cases of these, and extremal higher dimensional black strings also, give negative energy plane fronted waves. In fact, all of the duals of positive mass solutions that will be considered here have nonpositive energies, but an argument is given which suggests that this is not true in general. 
Dean L. Welch