[vsnet-obs 21035] Re: V382 Vel comp stars 56 &59

["Bruce Sumner" <b.sumner@bom.gov.au>]

	In reply to Berto's observation that on the V382 Vel chart 
comparison star 59 appeared brighter than 56.
	
	This observation nicely shows the pitfalls that bedevil a good 
variable star sequence. Both magnitudes are taken from Tycho, and are 
accurate, but....
	
	Here are the the V, B-V and derived v(visual) values for these two 
stars. v is derived from v=V+0.2*(B-V)
	 V    B-V     v     Comment
	5.56  1.56   5.87   very red
	5.90  0.17   5.93   =IW Vel, a low amplitude blue variable

	59=IW Vel apparently has an amplitude of about 0.02B, and a period 
of 0.14 days. This low amplitude should not preclude it from being 
used as a comparison star, but if possible no known variable should 
be used as a comparison star. 
	
	56 is a red star, and visually will appear fainter, how much depends 
on the individual observer. It does not make a good comparison star. 

	Thus Berto's observation that 59 appeared brighter than 56 is very 
plausible. The moral of this tale - comparison stars should not be 
variable stars and always try to use comparison stars with silmilar 
colours, i.e. don't mix red and blue stars in the same sequence.

Bruce Sumner

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