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

               AAVSO ALERT NOTICE 228 (September 6, 1996)


Astronomers at the University of Southampton, England, have requested our
assistance in their upcoming observations with the Hubble Space Telescope (HST)
of the eclipsing dwarf nova type cataclysmic variable HS 1804+6753.  Their
observations will be made on September 13 from 02:20 to 15:06 UT, and they
will be obtaining high time-resolution ultraviolet spectra of the system.
Their observing window will include two eclipses, with mid-eclipse occurring
at 05:49 and 10:51 UT.  Full coverage of an eclipse requires about one
hour on either side of mid-eclipse.

Please monitor HS 1804+6753 closely between now and the end of the month, and
report your observations to AAVSO Headquarters on a nightly basis, so that we
may inform the astronomers of the status of the system before, during, and
after the HST observations.  Observations in the AAVSO International Database
indicate that HS 1804+6753 varies from visual magnitude 12.5 to 16.0.

Observers with Charged-Couple Devices (CCDs) with or without filters are
strongly recommended to contribute to this important observing run.  The
astronomers have indicated that high speed CCD photometry, using B,V,R,I
filters, obtained during the time of the HST observations themselves would
be especially valuable. These multicolor data would be useful in extending the
wavelength range from space ultraviolet all the way to infrared and would
allow the astronomers to determine the luminosities and temperatures of the
different components of this compact binary system.   Observers are recommended
to make such observations at 3-minute or shorter intervals.  It is essential to
report the exact time of these observations to the second, if possible.

Accompanying is a chart of HS 1804+6753 made using The Sky software; the
magnitudes are from the Guide Star Catalog.  AAVSO longtime observer John
Bortle has sky-checked this comparison star sequence and indicates that he has
found it acceptable.

Note that the Designation 1804+67 (that is, the hours and minutes of right
ascension and degrees of declination for epoch 1900) is the same as the hours
and minutes of right ascension and degrees of declination for the 2000
position (1804+67).  The amount of precession from 2000 to 1900 is very small
because of the high latitude of the system:

         (2000)   R.A. = 18h 04m 13.9s    Decl. = +67 degrees 54' 11"

precesses to

         (1900)   R.A. = 18h 04m 35.5s    Decl. = +67 degrees 53' 32"


Electronic copies of the chart of HS 1804+6753 mentioned in this Alert Notice
are available from our FTP site:

     (, in /pub/alert228

The chart may also be accessed through our Web site at the following address:


The answering machine at AAVSO Headquarters is on nights and weekends for your
convenience.  Please call our charge-free number (800-642-3883) to report your
observations.  If you are cut off when you telephone in your observations,
please wait a few minutes and call back to complete your call.  We have learned
that if someone calls to leave observations on the answering machine and while
they are speaking someone else calls, the first person may be cut off.  We
also encourage observers to send observations by fax to 617-354-0665 or by
e-mail through the Internet to  We would appreciate it
very much if you would report your observations in Universal Time.

Many thanks for your valuable astronomical contributions and your efforts.

Good observing!

Janet A. Mattei

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