American Association of Variable Star Observers (AAVSO)
Tue, 01/23/2024 - 12:30

The next eclipse of the very long period binary is starting in 3 weeks.

Observations are requested: "31 Cyg is one of the three classical zeta Aur type eclipsing systems already observed by Olin Wilson in the 1950ies and Wellmann in the 1930ies, among other professional  astronomers. It presents a rare opportunity, where a relatively small B-star companion traces like a scanner an extended K supergiant  chromosphere before and after the main eclipse.

The next mid-eclipse is due March 22, 2024, observable in the morning hours. With a period of 10.4 years, the last observed 31 Cyg eclipse was in the summer of 2003.
This rare event will be covered by modern high-resolution spectroscopy with the TIGRE 1.2m robotic telescope and, certainly, other facilities.

Photometry of the partial phases, which last a bit over 2 days and fall in the periods of february 16-22 and april 20-26,  will help to establish the exact contact times and so provide a precise height-scale for the chromospheric analysis by any other observations.

Since colours change significantly in the event, measurements uncalibrated to Johnson filters can still serve very well, when the photometry includes good values inside and far from partial eclipse phases.

Klaus-Peter Schroder
Prof. of Astronomy
University of Guanajuato"

Observations of the eclipse magnitude were successful in 2013 using ordinary DSLR cameras. BAV report is here:

Currently I am observing with a Canon 600D and a lens 1:2.8 f=50mm. Exposure times of 10..13 seconds and ISO 200..400 work well for me. The usual kit lenses 18-55mm should also work well. My light curve is attached.

Clear skies,

Wolfgang Vollmann VOL


More data about this event ?

Hello EB observers,


I'm interested by those rare eclipse occurrences. Thanks to point out this one.

I will possibly try to observe it, weather permitting.


Anyway, I have a few questions about the coming and past 31 Cyg eclipse.


I could'nt find any documentation on it, apart this old paper.


In your alert, you wrote that the partial phase of the eclipse were lasting for about two days. But, I saw in the attached paper in  german, that the partial phase between contacts were lasting 12 days. Is there a typo or something like that somewhere ?

I also noted the amazing increase in brighness after the eclipse. Any explanation about this phenomenon ?

What were the comparison stars used to observe this 2013 eclipse ?


This could be a problem for me to find a good comp star nearby. Any recommandation ?


And, finally : are the observed timings of the 2003 eclipse (and other, if any) available somewhere ?


Thank you so much in advance,


American Association of Variable Star Observers (AAVSO)
some information on 31 Cygni

Hello Christophe,

great that you want to observe this rare event. Lets hope for clear skies!

As comparison and check stars I think that 30 Cyg nearby and 26 Cyg are the best what is available. This we also used back in 2013. I assume that the brightening after the 2013 eclipse comes from intrinsic variability of the K supergiant. Regarding the duration of partial phase I cited the days what the TIGRE team is most interested in. Of course observations will show.

Best regards