RX And B-V anomoly?

Just to be clear, you're talking about  RX And, right?

The B-V listed in VSX comes from APASS

APASS-DR9    B = 13.71   V = 13.43   g' = 13.53   r' = 13.31   i' = 12.65   B - V = 0.28

     This is a cv, so one could expect a fairly blue color.  But the problem with the Tycho-2 measure quoted by Wikipedia is that the uncertainty is omitted.  The star is mag 12-ish, so is right at the limit of detectability for Tycho-2.  The uncertainty on the B value is nominally 0.09 mag, while that for V is 0.21 mag.  'Real' Tycho-2 errors for faint stars (fainter than 9th) are in fact about 50% larger than this, so the implied B-V color is very uncertain.  VSX quotes the APASS B-V, whose uncertainty is 2.14 mag(!), which surely comes from the mean value including outburst as well as quiescent data (says 14 observations on three nights).  The Mermilliod UBV compilation quotes B-V = 0.00, but you'd have to look up the source for that.  If there are some A- or F-type stars in the field, you might select those as comps, but they are relatively uncommon, so you'll have to use whatever's available.  Since this is a well-observed object, I am surprised there is no 'official' AAVSO sequence, or that one exists in the literature.  SIMBAD shows no Henden biblio item for RX And, so evidently Arne did not get to this one as part of cv calibration papers he did 20+ years ago.

\Brian