Non-linear relativistic perturbation theory with two parameters
  An underlying fundamental assumption in relativistic perturbation theory is the existence of a parametric family of spacetimes that can be Taylor expanded around a background. Since the choice of the latter is crucial, sometimes it is convenient to have a perturbative formalism based on two (or more) parameters. A good example is the study of rotating stars, where generic perturbations are constructed on top of an axisymmetric configuration built by using the slow rotation approximation. Here, we discuss the gauge dependence of non-linear perturbations depending on two parameters and how to derive explicit higher order gauge transformation rules. 
Carlos F. Sopuerta, Marco Bruni (1) and Leonardo Gualtieri (2) ((1)   ICG, Portsmouth U. (2) Universit\`a di Roma ''La Sapie...