StefanUtt on 26-Jul-2011 13:44:12 GMT about V* BH Cru
The spectral type of the Mira variable BH Cru (S) is outdated. It was of spectral type S when it was observed spectroscopically for the first time in the 1960ies (see e.g. Catchpole & Feast, 1971, MNRAS 154, 197; there it is listed as Henize 120), but it changed its spectral type later on to SC and finally to C. The most recent spectroscopic observation of BH Cru is by Uttenthaler et al. (2011, see http://adsabs.harvard.edu/abs/2011A%26A...531A..88U), where it clearly is of spectral type C. The C subtype has not been determined in detail, but must be close to C8.2.
The reason for the change in spectral type is debated, but most likely it is due to a recent third dredge-up even that mixed additional 12^C to the surface of the star, thus increasing the C/O ratio to a value larger than one, and thus changing the spectral type. Such changes in spectral type are rare, but do occasionally happen, as is demonstrated in this case!
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