Citing this Article
 

Owing to the fact that a Living Reviews article may evolve over time, we recommend to cite the article as follows:

Timothy M. Adamo and Ezra T. Newman and Carlos Kozameh,
"Null Geodesic Congruences, Asymptotically-Flat Spacetimes and Their Physical Interpretation",
Living Rev. Relativity 15,  (2012),  1. URL (cited on <date>):
http://www.livingreviews.org/lrr-2012-1

The date given as <date> then uniquely identifies the version of the article
you are referring to.