Second-order symmetric Lorentzian manifolds II: structure and global   properties
  We give a summary of recent results on the explicit local form of the second-order symmetric Lorentzian manifolds in arbitrary dimension, and its global version. These spacetimes turn out to be essentially a specific subclass of plane waves. 
O F Blanco, M S\'anchez, J M M Senovilla
math.DG gr-qc