Keenan, Philip C. Spectral types and their uses. Publications of the Astronomical Society of the Pacific. August 1987, 99: 713–723. Bibcode:1987PASP...99..713K. doi:10.1086/132036.
Keenan, Philip C.; et al. Recognition and classification of strong-CN giants. Publications of the Astronomical Society of the Pacific. July 1987, 99: 629–636. Bibcode:1987PASP...99..629K. doi:10.1086/132025.
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Keenan, P. C.; Heck, A. SMR stars, strong-CN stars, and R stars. Revista Mexicana de Astronomía y Astrofísica. July 1994, 29: 103–110. Bibcode:1994RMxAA..29..103K.
Keenan, Philip C. Spectral types and their uses. Publications of the Astronomical Society of the Pacific. August 1987, 99: 713–723. Bibcode:1987PASP...99..713K. doi:10.1086/132036.
Keenan, Philip C.; et al. Recognition and classification of strong-CN giants. Publications of the Astronomical Society of the Pacific. July 1987, 99: 629–636. Bibcode:1987PASP...99..629K. doi:10.1086/132025.