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WZ Cassiopeiae.

Mr Abdul Ahad, in his letter in the December Journal (120(6), 374 (2010)) described his search, conducted bibliographically, for the double star with the greatest colour contrast. He recognised that the reddest stars are carbon stars, and from tabulated colour indices selected WZ Cas and its visual companion as having the greatest contrast of any pair, at least as far as magnitude 8.5.

His conclusion might be called into question on several grounds. First, the 'companion' of WZ Cas is just a field star almost a minute of arc away from WZ, so as a double star the pair is open to objection on the grounds of being merely optical. Secondly, the colour sensitivity of the human eye is greater for bright objects than for faint ones, so a bright pair such as Albireo or [gamma]. And may appear more strongly contrasted than a faint pair even though the objective difference in measured colour index may be smaller. Thirdly, the close juxtaposition of the components in many visual double stars such as [gamma] And and [epsilon] Boo enhances the apparent contrast. And finally, carbon stars are notoriously variable, not just in magnitude but in colour too, and it is probably no more than fortuitous that WZ Cas should happen to have one of the largest numerical values of (B-V) in the database consulted by Mr Ahad.

I could support some of these points from my own experience. As a schoolboy and then an undergraduate I was an enthusiastic member of the Variable Star Section, and I made visual observations of WZ Cas on nearly 250 nights in 1953-'57. It did not take long for me to realise that the fearful colour difference between the variable and any available nearby comparison star made for great difficulty in comparing them, so I adopted the scheme of using the smallest possible optical power (I had a number of very small telescopes) that sufficed to enable me to see the stars clearly. When it was near the limit of vision, the variable's colour was quite unobtrusive, so a magnitude comparison was readily made, even if its actual meaning may not have been too certain! Subsequently (1) I have had WZ Cas on a programme of radial-velocity measurements, mostly made at the Cambridge 36-inch telescope, and have observed it on more than 500 nights, so I have some familiarity with its appearance. I certainly notice how its colour, as well as its magnitude and radial velocity, fluctuates.

The connection between colour perception and apparent magnitude is well illustrated by the following anecdote. On 1983 October 16, with my collaborator J. E. Gunn, I observed WZ Cas for radial velocity with the instrument at the coude focus of the 200 inch reflector on Palomar Mountain, which a rough calculation shows to have something like a million times the light-grasp of the naked eye--a matter of fifteen magnitudes. (2) The star came into the field of the finding eyepiece such a blinding red colour, like a traffic light on 'stop', that the night assistant (to whom of course the telescope was a commonplace but who nevertheless had evidently never seen anything like that previously) felt constrained to put out an invitation to anybody who might be in the dome at the time to come and see it--and several people whose presence there was unsuspected did promptly arrive to share the astounding sight!

From Prof R. F. Griffin

R. F. Griffin

The Observatories, Madingley Road, Cambridge CB3

0HA. []

(1) T. Lebzelter, R. F. Griffin & K. H. Hinkle, A&A, 440, 295 (2005)

(2) R. F. Griffin & J. E. Gunn, ApJ, 191, 545 (1974)
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Title Annotation:Letters
Author:Griffin, R.F.
Publication:Journal of the British Astronomical Association
Article Type:Letter to the editor
Date:Feb 1, 2011
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