W Ursae Majoris-type eclipsing variables
U Geminorum-type variables, 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
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).
W Ursae Majoris-type eclipsing variables. These are eclipsers with periods usually shorter than 1
day, consisting of ellipsoidal components almost in contact and having light curves for which it is
impossible to specify the exact times of onset and end of eclipses. The depths of the primary and
secondary minima are almost equal or differ insignificantly. Light amplitudes are usually <0.8
mag. in V. The components generally belong to spectral types F-G and later.
Binary systems with at least one white-dwarf component, or a single rotating white dwarf.
AAVSO Alert 351-5/471: 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 16 November 2018