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.
Smith, G. R.; Harmer, D. L. (January 1982), "A differential curve-of-growth analysis of the candidate barium star 93 Her", Monthly Notices of the Royal Astronomical Society, 198: 273–280, Bibcode:1982MNRAS.198..273S, doi:10.1093/mnras/198.1.273.
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.
Smith, G. R.; Harmer, D. L. (January 1982), "A differential curve-of-growth analysis of the candidate barium star 93 Her", Monthly Notices of the Royal Astronomical Society, 198: 273–280, Bibcode:1982MNRAS.198..273S, doi:10.1093/mnras/198.1.273.