photometry on overlapping stars

Hi everyone:

I'm interested in doing photometry on a pair of stars that are very close, overlapping one another, and are both very bright (mag10) for the equipment I use.

Assume that both stars are varying.

AIP seems to be able to determine the centroid of the individual stars in this pair.  How should the annulus be set, specifically the middle and outer radii?  The outer ring (normally set to contain just background sky) will of course contain the companion star's light, so I'm unsure of how much/how little should be included.  My previous photometry has been on isolated stars so the annulus settings were straightforward.  

Very short exposures separate out the stars better, but then there's the problem of low SNR and flux issues.

Or--should I treat the pair of stars as single entity and then have the photometric data parsed out later--have software analysis determine the variability of each?

Thanks...

Steve Smith (SSTB)