Open Strings in Exactly Solvable Model of Curved Spacetime and PP-Wave   Limit
  In this paper we study the superstring version of the exactly solvable string model constructed by Russo and Tseytlin. This model represents superstring theory in a curved spacetime and can be seen as a generalization of the Melvin background. We investigate D-branes in this model as probes of the background geometry by constructing the boundary states. We find that spacetime singularities in the model become smooth at high energy from the viewpoint of open string. We show that there always exist bulk (movable) D-branes by the effect of electric flux. The model also includes Nappi-Witten model as the Penrose limit and supersymmetry is enhanced in the limit. We examine this phenomenon in the open string spectrum. We also find the similar enhancement of supersymmetry can be occurred in several coset models. 
Hiromitsu Takayanagi and Tadashi Takayanagi