Circular No. 7675

Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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WZ SAGITTAE
     D. Steeghs and T. Marsh, Southampton University; E. Kuulkers,
Space Research Organization Netherlands and Utrecht University; and
W. Skidmore, University of St. Andrews, on behalf of a larger
collaboration, report:  "Strong spiral waves are present in the
accretion disk of WZ Sge during the first week of its outburst
(IAUC 7669, 7670).  Phase-resolved spectroscopy, obtained between
July 28.02 and 28.24 UT with the 2.5-m Isaac Newton Telescope (+
CCD spectrograph) at La Palma, yielded 2411 spectra (resolution
5700) covering four binary orbits that were used to construct
Doppler tomograms showing the distribution of line emission in the
binary frame.  The He II (468.6 nm) and C III (464.7 nm) images
reveal that the accretion-disk emission is dominated by two spiral
arms.  The H-beta and He I (492.1 nm) lines, on the other hand, are
dominated by highly variable absorption cores.  This is the first
time that spiral arms have been observed in WZ Sge, making it also
the first low-mass-ratio cataclysmic variable that displays spiral
arms in its disk during outburst.  We encourage follow-up
spectroscopy to track the evolution of these structures."

                      (C) Copyright 2001 CBAT
2001 July 31                   (7675)            Daniel W. E. Green