Dilatonic Black Holes, Naked Singularities and Strings
  We extend a previous calculation which treated Schwarschild black hole horizons as quantum mechanical objects to the case of a charged, dilaton black hole. We show that for a unique value of the dilaton parameter `a', which is determined by the condition of unitarity of the S matrix, black holes transform at the extremal limit into strings. 
P.H. Cox, B. Harms and Y. Leblanc