D-branes and BCFT in Hpp-wave backgrounds
  In this paper we study two classes of symmetric D-branes in the Nappi-Witten gravitational wave, namely D2 and $S 1$ branes. We solve the sewing constraints and determine the bulk-boundary couplings and the boundary three-point couplings. For the D2 brane our solution gives the first explicit results for the structure constants of the twisted symmetric branes in a WZW model. We also compute the boundary four-point functions, providing examples of open string four-point amplitudes in a curved background. We finally discuss the annulus amplitudes, the relation with branes in $AdS_3$ and in $S^3$ and the analogy between the open string couplings in the $H_4$ model and the couplings for magnetized and intersecting branes. 
G. D'Appollonio and E. Kiritsis