(cba:news) Nemesis to WZ Sge...

[Joe Patterson]

Dear CBAers,

No, not the weather.  That too, maybe.  But now that WZ is fading a bit,
it's time to worry more about the nefarious little companion star.

All previous studies have had to do something about the companion, and we
do too.  If feasible, you could just exclude it from your aperture or psf
photometry.  In our photoelectric photometry from the 1970s, that's
what usually did.  The enclosed discussion by Arne Henden gives the
relevant numbers which indicate how to do this, and are also a measure of
the severity of the problem.

***************************************************
WZ Sge     20:07:36.53  +17:42:15.2  J2000
companion  20:07:35.77  +17:42:17.1  J2000

with typical errors of 100mas. This gives a separation of 10.9 arcsec.
Magnitudes of the companion alone:

  V     B-V    U-B    V-R    R-I
13.884  1.526  1.592  0.776  0.692

(0.03mag total errors except for U-B which has 0.07mag error).
You should not have problems with contamination as long
as your aperture is 12 arcsec in diameter or smaller.
I'd suggest either using a small aperture or else one
large enough to encompass both stars; the inbetween ranges
will have some weird results.  Remember, however, that the
contamination is highly dependent on your passband, worse at
red wavelengths or with unfiltered photometry.
***************************************

    Most of us observe unfiltered, which approximates an R passband,
implying that full contamination means an extra unwanted R=13.11 star.
Looking at the data you've sent, it's pretty easy for me to tell which
observers have cleanly excluded or included this star.  But in-between
cases are hard to identify, and will inevitably give noisy data since the
amount of contamination will depend drastically on the instantaneous
seeing.  Don't mess with Mister Inbetween (Sinatra 1942).

    Unless your telescope drive and/or seeing are really bad, I recommend
excluding the companion.  This is because background from sky and
companion will start to overwhelm you beyond about magnitude 13.5 - and
the light curves will suffer greatly.  We're interested in the detailed
light curves on the approach to quiescence.  A too-small aperture hurts
light curves too, but you'll be able to study those problems, gain
experience, and maybe even fix them... whereas a too-large aperture just
swamps everything in the ocean of Poisson noise.

    In any case, send me a note describing how you've extracted your
delta-magnitudes so far, and whether you can do OK with a box that
excludes the companion.  Our emphasis is time-series, so this is not the
most critical issue for us... but now's the time to report the details,
and find the right strategy for WZ Sge's fainter days ahead.

    Thanks, Arne, for your measurements, and for improving our attention
to this issue!


                    joe