Last minute update: IT WAS A FALSE ALARM.
A ZTF observation taken on December 3, 02.45 UT, shows the star is normally and slowly brightening to maximum at g= 14.95.
https://alerce.online/object/ZTF18ablvwmz
The star looked blue in the original report images, so we might be looking at a NIR leak.
Confusion arose because a provided image comparison was made with an archival image, and not with the same set up on consecutive nights as we had understood.
Sorry for the confusion.
AM Cyg is a 370 d. mira-type variable that ranges between visual magnitudes 10.5 and 15.3.
On the morning of December 1 was rising to V= 14.0 (latest observation from ZTF), but we have received a report from Charlie Worrick (Chatham, MA) indicating that the star may have brightened very quikcly during that day, reaching 8th magnitude over 21 hours and becoming bluer.
We need to rule out instrumental problems, but a quick inspection of the images did not reveal anything unusual with them, and AM Cyg seems to have indeed brightened.
The last AAVSO observation was made November 16, and the ASAS-SN database can’t be accessed right now, so we need confirmation from observers all around the world.
We can’t rule out a line-of-sight coincidence of a potential nova candidate (in that case the amplitude would be >12 mag.) in the background, but that seems highly unlikely.
Such a brightening in a mira-type variable wouldn’t be unprecedented. V0407 Cyg was a 768 d. mira and had a nova outburst in 2010. It turned out to be a symbiotic system. This one might be similar, but we need confirmation!
Please observe the field as soon as possible and report your observations to the AAVSO.
AM Cyg is at 20 49 00.85 +31 50 55.8 (J2000.0)
VSX entry of the star.
Clear skies!