Plane waves from double extended spacetimes

We study exact string backgrounds (WZW models) generated by nonsemisimple algebras which are obtained as double extensions of generic D-dimensional semisimple algebras. Even if a suitable change of coordinates always exists which reduces these backgrounds to be the product of the non-trivial background associated to the original algebra and two-dimensional Minkowski, under Inönü-Wigner contraction the algebra reduces to a Nappi-Witten algebra and the corresponding plane wave spacetime, no more factorized, is the Penrose limit of the original background. We construct the spectrum of D-branes for the double-extended background and study their behavior under the Penrose limit. While in general D-branes become singular in this limit, we prove that a class of solutions with a well-defined limit exists and it gives rise to the spectrum of configurations of the contracted model. Therefore, all D-branes of the plane wave background can be obtained as suitable contractions of original ones. We also discuss the Penrose limit at the quantum level and argue that the procedures of contraction and quantization commute.