(cba:news) FO Aquarii, keep the faith

Joe Patterson jop at astro.columbia.edu
Sat Nov 18 09:13:17 EST 2017

Hi CBAers,

Some of you still have FO Aqr in your observing programs - for 
occasional coverage - despite the unfavorable sky location.  For our 
time-series photometry, an equatorial star 6 hours out of season is 
usually a no-fly zone!  But this year is different.  The usual dominant 
signals are still present, but the mean light has been fading throughout 
the season.  Now at 14.9, and seemingly still diving.  Over the years it 
has averaged about 13.7, so it's not exactly a call-911 deal.  Since it 
was comparably low early in the previous season (2016), I'd call it a 
"somewhat low but somewhat unstable" state.  Accurate, but not real catchy.

In a CV, is this *significant*?  Hard to say, since most of the previous 
35 years (mostly ours) used differential photometry, so the constancy of 
mean light over this interval is only good to around +-0.4 mag.  I would 
have noticed anything more than that.  So it's somewhat significant... 
but the more interesting question is, is it useful?

The answer is YES.  In theory, the magnetic WD spins "up" (period 
decreasing) when in a high-accretion state, and spins down in a 
low-accretion state.  Of course, HOW LOW is an important and probably 
unanswerable question.  But we do the best we can!  In this case, we can 
track spin-up/down by studying the 21-minute signal, which is strong as 
ever.  The fainter the star, and the longer the duration of faintness, 
the more convincing is this test.

Northern observers are now favored, because of the long nights.  Another 
month of coverage would be great.  Our usual advice of "not more than 
2.0 airmasses" doesn't quite apply here, since we're tracking a 21 
minute signal.  On a clear night, you can probably stretch it out to 3.0 
airmasses.  Beyond that, you're almost certainly in the regime of "bad 
data driving out the good".

No one biting yet on DN Gem.  This is likely to be a real winner!

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