[Aavso-photometry] Comp star & target star colors

[Wolfgang Renz]

Hello Keith

If the comp star colors match the color of the variable star,
you will get just small color depentant residuals in filtered
photometry without color dependent transformation.

But its not so easy to find a constant red comp star for M or
even C or S type Miras, as all stars that are such red are all
more or less variable. So usually late K type stars are the
reddest stars that are used as comp stars. Mira star photometry
should definitly gain by applying color dependent transformation.

What kind of CVs are interested in ?
SN, Novae, DN, Symbiotic, ... ?
E.g. Z And stars are usually pretty red, ...

The light of DN are often dominated by the hot/bright spot in
quiescence and always by the disk during an outburst (the super
humps caused by the bright spot are usually < 0.3 mag). In these
systems the primary and secondary stars contribute often less
than 50% of the light even at quiescence. The red secondary
doesn't contribute much to the total light (no secondary eclipses
in eclipsing DN) while the eclipses of the primary white dwarf
are usually smaller than the eclipses of the hot/bright spot.

Brian Warner has in his 'Cataclysmic Varaible Stars' book a
chapter about 'Colour Variations' (3.3.4.1, p. 148 ff) of DN in
outburst. It states that DN with slow rises don't change color
much during the outburst cycle. But DN with rapid rises do
change their color. He gives a nice two-color diagram of VW
Hyi. In quiescence it has a B-V of ~ +0.1 mag that increases
to ~ +0.55 mag during the rise and back to -0.15 .. +0.1 at
maximum. During the fading (that is usually always slower
than the rise) it just changes color back to +0.1 mag.
So the assumption that DN usually have a B-V of ~ 0.0 is
correct during maximum, fading and quiescence but not
during the rise of fast rising DN.

Clear skies
  Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO


----- Original Message ----- 
From: "Keith Graham" <kag at core.com>
To: "aavso Photometry" <aavso-photometry at aavso.org>
Sent: Tuesday, March 15, 2005 2:32 AM
Subject: [Aavso-photometry] Comp star & target star colors


Hi,

With all of the discussion lately about matching comp star colors to the target
star color, a question has come to mind.
It would be fairly easy to match a comp star to a nice red Mira, however what
about CVs?

CVs characteristically consist of a main sequence star G, K, or M class star
orbiting a white dwarf (O to B class).
How would one go about matching comp stars to such a combination?

Cheers,
Keith Graham