[vsnet-alert 10903] Re: possible new cataclysmic variable in Ursa Major

[Brian Skiff]

     I notice that in this instance the V magnitude shown in the
NOMAD catalogue has actually been used for the photometry.  Yikes!
Please be aware that the photometry there is _not_ properly
calibrated data, and is often rather far from the standard system.
>From the same photographic plates, the GSC-2.3 shows V = 15.94
for the same star.  In addition, the Sloan digital sky survey (SDSS)
shows g = 16.07 and r = 15.64 for this star.  These figures suggest
the GSC-2.3 V mag is more nearly correct (but not necessarily
accurate!).  Thus the data for the variable based on NOMAD values
will have to be recalibrated.  It is unfortunate that the
astronomical literature can be misleading, but that is how it is.
     Perhaps for events like this, in the north galactic cap, 
folks should adopt SDSS photometry for field stars, which is good
to a couple percent from mag 15 down to mag 20 or 21.

\Brian



>  Results:
> YYYYMMDD                 mag                   Comments
> 20090104,215859    15.056 V      comp. star NOMAD 1379-0266557  Vmag=15.27

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