Exact String Solutions in Curved Backgrounds
  We show how the classical string dynamics in $D$-dimensional curved background can be reduced to the dynamics of a massless particle constrained on a certain surface whenever there exists at least one Killing vector for the background metric. Then we obtain a number of sufficient conditions, which ensure the existence of exact solutions to the equations of motion and constraints. The results are also relevant to the null string case. Finally, we illustrate our considerations with an explicit example in four dimensions. 
P. Bozhilov