Tension as a Perturbative Parameter in Non--Linear String Equations in   Curved Space--Time
  A perturbation theory with respect to tension parameter $\gamma/\alpha^\prime$ for the non--linear equations of string, moving in curved space--time is considered. Obtained are linearized motion equations for the functions of the $n-$th degree of approximation ($n=0,1,2$) 
A. A. Zheltukhin