[Aavso-photometry] Eclipsing binary magnitude ranges on AAVSO charts

[arne]

Yenal Ogmen wrote:
> Hi all,
> 
> I would like to know if the magnitude ranges of EBs on AAVSO charts are flexible or not. Because, my last night observation of ST PER eclipse showed ~12.13 v mag of the star at minimum, where on the chart it writes Mag: 9.5-11.4
> 
> Thank you in advance.
> 
>  
These values are either from the GCVS, or updated values from submitted
revisions.  You have to be a little careful, and make sure the wavelength
matches the passband of your filter.  In this case, VSX says that the
range 9.5-11.4 is standard V.  You are right to question the amplitude,
as it is not always correct - there are lots of errors in the GCVS.

The best way to see if your observations are reasonable is to first
look at the AAVSO submitted observations via the light curve generator.
This doesn't help greatly for ST Per, though with a 4000-day window,
you see a number of observations at maximum (V=9.5), plus a few
eclipses going down to V=12.5 or so, plus a bunch of differential
measures near zero that you have to ignore.  So you have an initial
hint that perhaps the GCVS magnitude range is incorrect.

If you then look at Simbad's references, you can find some detailed
studies of the system.  A particularly good one is 	
Srivastava, R. K., "Photoelectric elements of the eclipsing binary
ST Persei," 1970BAICz..21..219S

In there, the author gives light curves in B&V, relative to the
selected comparison star (BD +38 607; TYC 02851-01660;
03:00:16.79, +39:27:12.1). It shows the differential V range to be
-0.3 to +1.5, or 1.8 magnitudes.  If we assume V=9.5 at maximum,
this would mean that the star should be 11.3 at minimum,
which is why the GCVS gives the range it does.  The range is much
more in B, by the way - an amplitude of 2.4 magnitudes.

So - the question then becomes: are your observations "wrong", giving
a wider range, or has there been a geometric change in the orbit?
An example of this latter would be apsidal motion if the orbit were
eccentric, yielding a greater fraction of eclipse.  If so, you
could be heading towards a total eclipse one of these days.
Or, it might be that you are actually using a B filter.

An interesting case that deserves some more attention.
Arne