We study exact string backgrounds (WZW models) generated by nonsemisimple
algebras which are obtained as double extensions of generic D--dimensional
semisimple algebras. We prove that a suitable change of coordinates always
exists which reduces these backgrounds to be the product of the nontrivial
background associated to the original algebra and two dimensional Minkowski.
However, under suitable contraction, the algebra reduces to a Nappi--Witten
algebra and the corresponding spacetime geometry, no more factorized, can be
interpreted as the Penrose limit of the original background. For both
configurations we construct D--brane solutions and prove that {\em all} the
branes survive the Penrose limit. Therefore, the limit procedure can be used to
extract informations about Nappi--Witten plane wave backgrounds in arbitrary
dimensions.Comment: 27 pages, no figures, references adde
