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                    25 Birch Street, Cambridge, MA 02138 USA
                   Tel. 617-354-0484       FAX 617-354-0665

                    AAVSO ALERT NOTICE 257 (April 28, 1999)


We are collaborating with astronomers from Lawrence Livermore National
Laboratory, Livermore, CA, in the scheduling of multiwavelength
simultaneous observations, particularly with the Hubble Space Telescope's
(HST) Imaging Spectrometer (STIS)  and the Rosse X-Ray Timing Explorer
(RXTE), and possibly the Extreme Ultraviolet Explorer (EUVE) and
ground-based resources, of SS Cyg during outburst. One of the goals of this
observing run is to carry out a multiwavelength search for very
small-amplitude and very short-period oscillations of the white dwarf. 

A similar, highly successful campaign on SS Cygni was carried out in 1996
(see AAVSO Alert Notices 221 and 230 for information on this campaign; the
campaign and results are summarized in the Annual Report of the Director
in Volume 25, Number 2, of the Journal of the AAVSO).  However, although
EUVE and RXTE satellites were scheduled and had very successful runs, HST
could not be scheduled at that time.  In 1997 a second campaign was
scheduled (see AAVSO Alert Notice 241), but SS Cygni unfortunately did not
have suitable outbursts during times when HST observations could be

The present campaign is extremely important, as it is the LAST CHANCE for
these astronomers to observe SS Cygni with HST under this proposal.  Their
observing window closes June 30. 

In this year's campaign, the investigating astronomers have requested that
we inform them immediately when SS Cyg starts to go into outburst, i.e.,
brightens, and to continue to keep them informed throughout the outburst,
particularly during the rise and at maximum so that the satellites and
other observing resources may be scheduled to observe SS Cyg.  Timely
notification of the onset of the outburst and up-to-the-minute information
on the behavior of SS Cyg throughout the outburst are essential for these
multiwavelength observations. 

Presently, SS Cyg is near minimum, at approximate visual magnitude 11.5,
according to observations received at the AAVSO.  Its last outburst, a
narrow one, occurred in late March 1999, when it began to brighten from
magnitude approximately 11.8 on March 19-20, reached magnitude 8.3 on
March 24, and declined to magnitude approximately 11.5 by April 4, where
it has remained to date.  We predict the next outburst to occur sometime
between now and mid-May.  SS Cyg has had several "minioutbursts" between
real outbursts recently, when the star brightens to about magnitude 10.5 -
11 and fades to minimum after several days.  Your continuous observations
and alerts as the star brightens will be crucial to determining the type
of the next outburst. 

AAVSO "b", "c", "d", and "e" scale standard charts for SS Cyg have been
posted on our ftp site; see below for details.  Those needing paper copies
of these charts should contact AAVSO Headquarters. 

Please monitor SS Cyg closely, and inform us immediately by phone, fax, or
email when it starts to brighten, i.e., becomes brighter than 11.5.  Once
SS Cyg goes into outburst, please send in your observations daily so we
may keep the astronomers informed of the star's behavior and they may
schedule the observations.  The success of these observations very much
depends on your observations and the early alerts and continuous
information we can provide to our colleagues.  And remember - this is the
last chance for these astronomers to observe SS Cyg with HST under their
current proposal. 


Electronic copies of the AAVSO charts mentioned in this Alert Notice, as
well as the reversed versions prepared by Marc Biesmanns, are available
through our web site at the following address: 


These charts and the image may also be obtained directly from our FTP

    (, in /alerts/alert257) 

The answering machine at AAVSO Headquarters is on nights and weekends for
your convenience.  Please call our charge-free number (888-802-STAR =
888-802-7827)  to report your observations. We also encourage observers to
send observations by fax to 617-354-0665 or by e-mail through the Internet

Many thanks for your valuable astronomical contributions and your efforts. 

Good observing! 

Janet A. Mattei Director

AAVSO 49 Bay State Rd. Cambridge, MA 02138 617-354-0484