Gravitational lens under perturbations: Symmetry of perturbing   potentials with invariant caustics
  When the gravitational lensing potential can be approximated by that of circularly symmetric system affected by weak perturbations, it is found that the shape of the resulting (tangential) caustics is entirely specified by the local azimuthal behaviour of the affecting perturbations. This provides a common mathematical groundwork for understanding problems such as the close-wide (d <-> 1/d) separation degeneracy of binary lens microlensing lightcurves and the shear-ellipticity degeneracy of quadruple image lens modelling. 
Jin H. An (IoA, Cambridge)