[vsnet-chat 4799] HdC Stars

["Peter F Williams" <pfwilliams@onaustralia.com.au>]

There was recent mention of hydrogen deficient carbon stars (HdC stars) by
Sebastian Otero in a discussion on Upsilon Sgr (including vsnet-chat
4747/4749).  Since reading David Kilkenny's paper Recent Work on R Coronae
Borealis Stars in the 1992 book Variable Star Research: An International
Perspective, I have been following several of the brighter HdC stars.  This
is based on Kilkenny's suggestion the HdC stars may undergo rare deep
minima similar to the RCB stars but which have simply not been observed. 
XX Cam is mentioned as an example and this star has been observed to fade
only twice in the past 100-odd years.

The three HdC stars I have been following are easily observed visually with
binoculars and thus do not add greatly to my observing routine.  The
results can be summarised as follows:

HM Lib = NSV7085 = HD137613
Commenced observing 1995 April.
Observed range 7.4 to 7.9

LV TrA = NSV7820 = HD148839
Commenced observing 1997 May
Observed range 8.0 to 8.8

NSV11960 = HD182040 (no gcvs designation?)
Commenced observing 1995 September
Observed range 6.9 to 7.2

None of these have shown any indication of a deep decline during the
observed period.  However, some of the confirmed RCB stars do not do much
better.  UX Ant, for example, is currently recovering from its first 
decline since 1990 while Y Mus has had no major decline since I commenced
observing it in 1987 and appears to have been at maximum since at least
1980 according to circulars of the VSS RASNZ.  V854 Cen on the other hand
has been very active with 14 major events observed since regular monitoring
commenced in 1986.

If the HdC stars do experience deep minima they appear to be rare events
indeed while the RCB stars clearly have differing rates of activity.

Regards

Peter Williams
Heathcote NSW

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