[Aavso-photometry] U Gem color

[arne]

Walt Cooney wrote:
> With some favorable weather, I've been able to get five nights of BVRI 
> photometry of U Gem since the outburst started.  One thing my data shows 
> is that it was "colorless" near peak but is now becoming redder as it 
> heads to quiescence.  Is that typical behavior for dwarf novae?
> 
Each system is different, though the overriding census is "colorless
during outburst."  Most CVs are very close to the Sun, typically less
than a few hundred parsecs, so interstellar extinction is minimal.
Any color change is inherent in the system, and if the CV becomes
red, that is an intrinsic red color.  Sometimes it is because we
are seeing the donor star, sometimes it is cyclotron radiation;
sometimes it is a temperature difference between the white dwarf
and the accretion disk.

A simple (though laborious) way to compare CVs at quiescence is to
look at Bruce Sumner's sequences taken from my CV photometry, as he
gives magnitude and (B-V) for each star.  These are available from
the chart pages.  For the low-accretion-rate systems, the SDSS survey
contains magnitude and colors in quiescence; see the 5 (6?) paper
series by Szkody et al.

My photometry of U Gem shows that it is about (B-V)=0 in outburst
and about (B-V)=0.14 in quiescence, still pretty blue, and the
color change is probably just a small temperature difference.
Arne