U Geminorum-type variables are quite often
called dwarf novae. They are close binary systems
consisting of a dwarf or subgiant K-M star that fills
the volume of its inner Roche lobe and a white
dwarf surrounded by an accretion disk.
Orbital periods are in the range 0.05-0.5 days.
Usually only small, in some cases rapid, light
fluctuations are observed, but from time to time
the brightness of a system increases rapidly by
several magnitudes and, after an interval of
from several days to a month or more, returns
to the original state.
Intervals between two consecutive outbursts for
a given star may vary greatly, but every star is
characterized by a certain mean value of these
intervals, i.e., a mean cycle that corresponds to
the mean light amplitude. The longer the cycle,
the greater the amplitude.
These systems are frequently sources of X-ray
emission. The spectrum of a system at minimum
is continuous, with broad H and He emission lines.
At maximum these lines almost disappear or become shallow absorption lines. Some of these
systems are eclipsing, possibly indicating that the primary minimum is caused by the eclipse of a
hot spot that originates in the accretion disk from the infall of a gaseous stream from the K-M star.
According to the characteristics of the light changes, U Gem variables may be subdivided into
three types: SS Cyg-type (UGSS), SU UMa-type (UGSU), and Z Cam-type (UGZ).
AAVSO Alert 351-4/471: September 20, 2012
Drs. Boris Gaensicke (Warwick University), Joseph Patterson (Columbia University, Center for
Backyard Astrophysics), and Arne Henden (AAVSO), on behalf of a consortium of 16 astronomers,
have requested the help of AAVSO observers in monitoring the ~40 cataclysmic variables in the
table below in support of Hubble Space Telescope observations in the coming months.
The HST COS (Cosmic Origins Spectrograph) will be carrying out far-ultraviolet spectroscopy of
~40 CVs sequentially, with the aim to measure the temperatures, atmospheric compositions,
rotation rates, and eventually masses of their white dwarfs.
The primary purpose of the monitoring is to know whether each target is in quiescence
immediately prior to the observation window; if it is in outburst it will be too bright for the HST
Based on the information supplied by the AAVSO the HST scheduling team will make the decision
(usually) the evening before the scheduled observing time as to whether to go forward with the
Hills Observatory: 1 January 2013 to 15 September 2018