I know the odds of this would be very low, but I always love exploring edge cases. So what happens if you want to observe a star that’s variable but it’s only surrounded by other variables or no nother reliable comparison stars… What would you use for the baseline then?
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You contact the sequence team and ask nicely for them to create a sequence for your star! 
Extremely rare, almost impossible I would type, to not be able to get a sequence these days at least for B & V. 
If the sequence team can’t create a sequence then you are on your own I guess! 
Jim (DEY)
Yes, in my several years on the ST, I can recall few occasions when there were no (or too many!) comp stars. Currently I am doing a sequence request where the field contains only 3 comps, with none close to the magnitude of the target (it’s a low-amplitude DSCT) so on these occasions we ask the requester if they want to go ahead. Sometimes they don’t. I can remember doing a sequence for a star very close to M42 where you have more variables than constant stars. In that case I had to resort to obtaining the distances of certain stars. If their distances were close to that of M42 (say about 1200 ly) then they were rejected, Closer ones would be OK as they were less likely to be potential YSOs, not being associated with the nebula.
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It is usual problem, when star is too bright and FOV is too small.
There are two solutions:
- change the target;
- change the instrument.