Keenan, Philip C.; McNeil, Raymond C. (1989). "The Perkins catalog of revised MK types for the cooler stars". Astrophysical Journal Supplement Series. 71: 245. Bibcode:1989ApJS...71..245K. doi:10.1086/191373.
Luck, R. Earle (March 1994). "Open Cluster Chemical Composition. I. Later Type Stars in Eight Clusters". Astrophysical Journal Supplement. 91: 309. Bibcode:1994ApJS...91..309L. doi:10.1086/191940.
Keenan, Philip C.; McNeil, Raymond C. (1989). "The Perkins catalog of revised MK types for the cooler stars". Astrophysical Journal Supplement Series. 71: 245. Bibcode:1989ApJS...71..245K. doi:10.1086/191373.
Luck, R. Earle (March 1994). "Open Cluster Chemical Composition. I. Later Type Stars in Eight Clusters". Astrophysical Journal Supplement. 91: 309. Bibcode:1994ApJS...91..309L. doi:10.1086/191940.