Tachyon Effects on the 2-Dim Black Hole Geometry
  We study solutions of the tree level string effective action in the presence of the tachyon mode.We find that the 2-dim. static black hole is stable against tachyonic perturbations.For a particular ansatz for the tachyon field we find an exact solution of the equations of motion which exhibits a naked singularity.In the case of static fields we find numerically that the full system has a black hole solution,with the tachyon regular at the horizon. 
G.A. Diamandis, B.C. Georgalas and E. Papantonopoulos