Perturbative String Dynamics Near the Photon Sphere
  String dynamics near the photon sphere in Schwarzschild spacetime is considered on the basis of a perturbative approach with respect to a rescaled string tension as a small parameter. The perturbative string solution in the zeroth and first approximation is presented. The perturbative solution corresponds to a small deformations of the photon sphere in Schwarzschild spacetime. 
Mariusz P. Dabrowski and Alexander A. Zheltukhin