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While answering the question "Why have brown dwarf classes been dubbed L, T and Y?" I noticed that in the paper Kirkpatrick et al. (1999) "Dwarfs Cooler than 'M': The Definition of Spectral Type 'L' Using Discoveries from the 2 Micron All-Sky Survey (2MASS)", the letter "J" was ruled out as being in use as a standard carbon star type.

So far I have not seen a source that uses "J" as a carbon star type by itself, I've only seen it in combination with "C", e.g. "C3,3J" or "C-J4 C2 5" (both from XHIP). This contrasts with the other carbon star types "R" and "N" which have definitely been used as separate identifiers.

Has "J" ever been used as a stand-alone spectral type, e.g. "J3"?

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As far as I can tell, the J-type was introduced by Bouigue (1954), and have subsequently often called "J-type stars" or "J-stars", e.g. Abia & Isern (2000). This likely led to the J-type designation being put as "in use", though the spectral types themselves appear to always have been used in combination with "C".

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