[Aavso-photometry] Identifying a bad (varying) comparison star when doing ensemble photometry

[Lionel Catalan]

I have collected photometry data on LPH058 (a high mass X-ray binary in
Monoceros) using ensemble photometry with 5 comparison stars and a check
star. For several months, the check star was constant (+ or - 0.015 mag).
However, over a period of a few days, the check star suddenly dropped in V
magnitude by 0.04 and in I magnitude by about 0.1. I suspect that the
problem is not with the check star itself, because the calculated magnitudes
of the comparison stars (using Maxim DL photometry tool) also varied during
these few days. So, I suspect that one or more of the comparison stars is
actually varying. Can somebody suggest the most effective method of finding
which one(s)? 
 
Lionel Catalan CTE