VSX lists DY Per as type: DYPER. Shouldn't it be RCB?
Jim Roe [ROE]
'DY Per type' is a term which seems to be in general use these days to describe a possible subgroup to the RCB stars. I guess technically it should be classed as type SRB, as this is how it is (still) classified in the GCVS, although it very clearly isn't of that type.
DY Per stars are generally cooler that classical RCB stars, but both are rich in Carbon (although I do believe that hydrogen deficiancy has only been seen in DY Per itself?). The fades are slower than RCB type stars with a recovery which is quite symmetrical to the fade. More are being discovered in our own galaxy as well as the Magellanic clouds.
DY Per just happenes to be my favourite star!
DYPER is a sub-type of the very rare RCB stars. They seem to be the low-temperature, lower luminosity analogues of the "classical" RCB types, which are thought to be the result of a merger of white dwarf companions in a double star. SNIa supernove may result from mergers like that. There's a real interesting paper by Tisserand et al, in A&A and available at http://arxiv.org/abs/0905.3224, that is all about the connection between RCBs, DYPERs, and carbon stars. The abstract alone clarified the difference between the two types of RCB stars and whetted my appetite for more DY Per.
There are more DY Per stars being disovered these days and they seem to be more numerous than the classical RCB stars. Not as much is known about the DYPER light curves as the classical RCB stars, so I enjoy observing the DYPER stars more. I like the little-known, weirder variables best!
I'm not quite sure that DYPers are a "subtype" of RCB stars. They just show deep fadings too, although not so deep, but the most likely explanation for their dimmings are ejection events in normal carbon giants. So if the cause of the event is so different, I wouldn't call it an RCB subtype.
Also, they might be more numerous than the RCB stars but they aren't up to now. They are less luminous so that may be playing a role against their discovery