  We obtain a family of type IIB superstring backgrounds involving
Ramond-Ramond fields in ten dimensions starting from a heterotic string
background with vanishing gauge fields. To this end the global $SL(2,R)$
symmetry of the type IIB equations of motion is implemented as a solution
generating transformation. Using a geometrical analysis we show that the type
IIB backgrounds obtained are solutions to all orders in $\alpha^{\prime}$.
