We investigate the evolutionary status of four stars : V348 Sgr , DY Cen and MV Sgr in the Galaxy and HV 2671 in the LMC .
These stars have in common random deep declines in visual brightness which are characteristic for R Coronae Borealis ( RCB ) stars .
RCB stars are typically cool , hydrogen deficient supergiants .
The four stars studied in this paper are hotter ( T _ { eff } = 15-20 kK ) than the majority of RCB stars ( T _ { eff } = 5000-7000 K ) .
Although these are commonly grouped together as the hot RCB stars they do not necessarily share a common evolutionary history .
We present new observational data and an extensive collection of archival and previously-published data which is reassessed to ensure internal consistency .
We find temporal variations of various properties on different time scales which will eventually help us to uncover the evolutionary history of these objects .
DY Cen and MV Sgr have typical RCB helium abundances which excludes any currently known post-AGB evolutionary models .
Moreover , their carbon and nitrogen abundances present us with further problems for their interpretation .
V348 Sgr and HV 2671 are in general agreement with a born-again post-AGB evolution and their abundances are similar to Wolf-Rayet central stars of PN .
The three Galactic stars in the sample have circumstellar nebulae which produce forbidden line radiation ( for HV 2671 we have no information ) .
V348 Sgr and DY Cen have low density , low expansion velocity nebulae ( resolved in the case of V348 Sgr ) , while MV Sgr has a higher density , higher expansion velocity nebula .
All three stars on the other hand have split emission lines which indicate the presence of an equatorial bulge but not of a Keplerian disk .
In addition , the historical light-curves for the three Galactic hot RCB stars , show evidence for a significant fading in their maximum-light brightnesses of \sim 1 mag over the last 70 yr. From this we deduce that their effective temperature increased by a few thousand degrees .
If V348 Sgr is a born-again star , as we presume , this means that the star is returning from the born-again AGB phase to the central star of PN phase .
Spectroscopically , no dramatic change is observed over the last 50 years for V348 Sgr and MV Sgr .
However , there is some evidence that the winds of V348 Sgr and DY Cen have increased in strength in the last decade .
HV 2671 , located in the LMC has not been analyzed in detail , but at 5-Å resolution is almost identical to V348 Sgr .
Through the bolometric correction derived for V348 Sgr and the known distance , we can estimate the absolute V magnitude of HV 2671 ( M _ { V } = –3.0 mag ) and its bolometric luminosity ( \sim 6000 L _ { \odot } ) .