The spectral type assigned to carbon stars in the Morgan–Keenan classification, with subdivisions C0–C9. In the previous Harvard system these stars were classified as types R and N; those types are now classified respectively as C0–C4 and C5–C9. The C0–C4 subtypes follow a temperature sequence, but the differences in C5–C9 spectra are more closely linked to composition, particularly the carbon–oxygen ratio. Luminosity classes are difficult to assign to C stars, but an alternative designator is often used, based on the strength of the C₂ (Swan) band at 474 nm, and written as a subscript in the range 1–5; for example, Y Canum Venaticorum has a spectral type C5₄.