@incollection{AST_1985__S131__401_0,
     author = {Trautman, Andrzej},
     title = {Optical structures in relativistic theories},
     booktitle = {\'Elie Cartan et les math\'ematiques d'aujourd'hui - Lyon, 25-29 juin 1984},
     series = {Ast\'erisque},
     pages = {401--420},
     publisher = {Soci\'et\'e math\'ematique de France},
     number = {S131},
     year = {1985},
     mrnumber = {837209},
     zbl = {0599.53055},
     language = {en},
     url = {http://www.numdam.org/item/AST_1985__S131__401_0/}
}