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Tombaugh, C. W. (1937). "The Great Perseus-Andromeda Stratum of Extra-Galactic Nebulae and Certain Clusters of Nebulae Therein as Observed at the Lowell Observatory". 49 (291). Astronomical Society of the Pacific. ss. 259-263. Bibcode:1937PASP...49..259T. doi:10.1086/124840.