Higher Order Correction to the GHS String Black Hole
  We study the order $\alpha'$ correction to the string black hole found by Garfinkle, Horowitz, and Strominger. We include all operators of dimension up to four in the Lagrangian, and use the field redefinition technique which facilitates the analysis. A mass correction, which is implied by the work of Giddings, Polchinski, and Strominger, is found for the extremal GHS black hole. 
Makoto Natsuume