(cba:news) BT Mon and the nova project (Joe Patterson) [2017-12-05T02:50:33Z]

Hi CBAers,

Just to remind, BT Mon is a 1939 nova which shows deep eclipses with an 
8.01 hour period.  There are no published orbital light curves - because 
of the coincidence with the Earth's spin period.  But we can do it, 
because we span a range of longitudes.  In addition, it looks now like 
the star's period is changing on a surprisingly short timescale... so 
many observations around mid-eclipse would be nice.  (We need more than 
a few, because it's quite faint at mid-eclipse - so some averaging is 
needed.)

The eclipses are now occurring around JD xxxxx.72, so they're visible at 
eastern USA longitudes, and will be in the western USA in about a month 
(as the "0.01" part gradually accumulates!).  That means it will soon be 
visible also in Australia, where we have two great observers (Gordon 
Myers and Greg Bolt).

Not in Europe - but we need coverage of the other half of the orbit too 
(the part no published work has ever covered).

Part (most?) of the reason I'm keen on this is that the eclipse timings 
*so far* indicate a large decrease in the orbital period.  Conventional 
CV evolution theory has the period decreasing pretty fast around P = 8 
hours - but it has never been observed.  It probably takes an eclipsing 
system - and one with a 30-year baseline - to test it, because only such 
a star would offer the needed precision.  There are only ~3 such stars, 
and BT Mon is one.  Now let's see how it goes.

BTW this is a star where you can't really cheat the "<2 airmasses" rule 
of thumb.  As a star of long Porb, BT Mon's significant variations 
(eclipse-related and other) are slow-ish... and that means they're too 
easily confused with differential-extinction effects.

OK, maybe on good nights you can cheat a *little*... at least if you 
list the airmass in your data submission (which enables me to 
approximate a correction for differential extinction).  But watch out.

joe

p.s. also, fear not if your signal-to-noise at mid-eclipse is abysmal. 
The time of med-eclipse is really determined by the data approaching and 
following mid-eclipse.  Mid-eclipse itself is flattish and uninformative.
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