On the Black-Hole Conformal Field Theory Coupled to the Polyakov's   String Theory. A Non Perturbative Analysis
  We couple the 2D black-hole conformal field theory discovered by Witten to a $D-1$ dimensional Euclidean bosonic string. We demonstrate that the resulting planar (=zero genus) string susceptibility is real for any $0\leq D \leq 4$. 
M. Martellini, M. Spreafico and K. Yoshida