Classics Illustrated: Limits of Spacetimes
  We carefully study the $e \rightarrow m$ and $e \rightarrow 0$ limits of the Reissner-Nordstr\"om spacetime using Geroch's definition of limits of spacetimes. This is implemented by embedding the one-parameter family of spacetimes in anti-de Sitter space, and as a result we obtain metrically correct Penrose diagrams. For $e \rightarrow m$ two distinct limits are studied. 
Ingemar Bengtsson, S\"oren Holst, Emma Jakobsson
gr-qc