I think V2491 Cyg is around 14th mag now - but have a look and see. The 
color is pretty red (from interstellar absorption), and so you could 
consider a red filter.  When it gets much fainter, you'll be driven to 
unfiltered observation... and it's even possible that's required now.
As usual, our interest is the TIME SERIES (to study for periodic 
signals) - but if you do use a standard filter, observe a standard star 
so we can calibrate the observation.

Good luck with this one!

joe