Shapiro, Irwin I.; Pettengill, Gordon H.; Ash, Michael E.; Stone, Melvin L.; Smith, William B.; Ingalls, Richard P.; Brockelman, Richard A. (1968). «Fourth Test of General Relativity: Preliminary Results». Physical Review Letters. 20 (22): 1265–1269. Bibcode:1968PhRvL..20.1265S. doi:10.1103/PhysRevLett.20.1265
Shapiro, Irwin I.; Pettengill, Gordon H.; Ash, Michael E.; Stone, Melvin L.; Smith, William B.; Ingalls, Richard P.; Brockelman, Richard A. (1968). «Fourth Test of General Relativity: Preliminary Results». Physical Review Letters. 20 (22): 1265–1269. Bibcode:1968PhRvL..20.1265S. doi:10.1103/PhysRevLett.20.1265