Taken from:
Unexpected Benefits
(Preliminary results from research by
the AAVSO Chart and Sequence Database Teams) -JAAVSO 2004, in preparation
Mike Simonsen, C. E. Scovil Observatory, Imlay City, MI, USA
Bruce Sumner, AAVSO Chart Team, Gex, France
Arne Henden, Universities Space Research Association/ U.S.
Naval Observatory, Flagstaff, AZ, USA
Vance Petriew, AAVSO Chart Team, Regina, SK, Canada
U Gem is a prototypical dwarf nova discovered by English
astronomer J. R. Hind in 1855. As such, it has an extremely long history of observations and has garnered attention from professional
and amateur astronomers around the world for over 150 years.
Several stars in the field of U Gem, historically used
as comparisons at one time or another, have come to be suspected of being variable themselves, in particular, NSV 3797 (GSC
1375-01085), NSV 3808 (GSC 1375-00893), and NSV 3816 (GSC 1375-01174).
Stars that are themselves variable cannot be used as
comparison stars in any valid comparison star sequence. In most cases, when determining ideal candidates to use as comparison
stars in a new or revised sequence, the sequence maker is generally inclined to avoid suspected variables. These may show
up as constant stars in a limited number of observations in a standard system, but this does not specifically rule out the
possibility of variability. Since these particular suspected variables are conveniently located in the field of U Gem, and
would, in most cases, be ideal comparison stars for the brighter end of a new sequence, they were investigated along with
the other field stars for which reliable photometry was available as suitable comparisons. Data from ASAS-3 (Pojmanski 2002)
and the Northern Sky Variability Study, NSVS (Wozniak et al 2004) were used to determine the constancy or variability of these
stars. They were all found to be constant with reliable V magnitudes from ASAS-3 data.
NSV 3797, NSV 3808 and NSV 3816 are all constant stars
with reliable magnitudes of 10.88+/- 0.03, 11.98+/- 0.04 and 9.17 +/- 0.01 respectively (ASAS-3). Note these are internal
RMS errors from the ASAS catalog and can have external errors larger than this.
GSC 1375-00447, (J2000) 07 55 12.1 +22 03 53, which
has been used historically as a comparison star by the AAVSO and other variable star organizations and observers, is a red
semi-regular variable with a range of at least 11.6-12.2V and a period of approximately 63 days. This star should not be used
as a comparison star when observing U Gem.
