I was just examining some new images of V5666 Sgr, and I discovered a small planetary nebula that is partially blended with the 12.6 comp star for the nova. The PN appears brightest in the V band. This star should not be used as a comp star.
This is PN H 1-67, or Hen 2-398. Nice catch! It shows up on the 5-second exposures from OC61. I agree that another comp star should be chosen.