Hazard, C.; Mackey, M. B.; Shimmins, A. J. (1963). Investigation of the Radio Source 3C273 by the method of Lunar Occultations. Nature197 (4872): 1037. DOI: 10.1038/1971037a0.
Schmidt, M. (1963). 3C 273 : A Star-Like Object with Large Red-Shift. Nature197 (4872): 1040. DOI: 10.1038/1971040a0.