What is causing the period variations in post-common envelope binaries?

"Origin of apparent period variations in eclipsing post-common-envelope binaries"
M. Zorotovic, M. R. Schreiber
http://arxiv.org/abs/1211.5356

There are several issues with the planet hypothesis for the post-common envelope binaries showing period variations (e.g. HW Vir, NN Ser), especially if the planets are assumed to be first-generation.

It may be that these are second-generation planets (or initially Neptune-sized planets that accreted matter when the progenitor star shed a large part of its mass), or it could be that these variations are intrinsic to the star and not related to planets at all.

Food for thought...