K-Models and Type IIB Superstring Backgrounds
  A family of type IIB superstring backgrounds involving Ramond-Ramond fields are obtained in ten dimensions starting from a K-model through a generalization of our recent results. The unbroken global $SL(2,R)$ symmetry of the type IIB equations of motion are implemented in this context as a solution generating transfromation. A geometrical analysis, based on the tensor structure of the higher order $\alpha^{\prime}$ terms in the equations of motion, is employed to show that these backgrounds are exact. 
Alok Kumar and Gautam Sengupta