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                AAVSO ALERT NOTICE 268 (December 20, 1999)

1216+05 SUPERNOVA 1999gn IN NGC 4303 = M61

We have been informed by the Central Bureau for Astronomical Telegrams (IAU
Circular 7335 - http://cfa-www.harvard.edu/iau/cbat.html) that Alessandro
Dimai, Cortina, Italy, discovered an apparent supernova in NGC 4303 (= M61) on
Dec. 17.22 UT at unfiltered CCD magnitude 16.0, using a 0.50-m telescope.
Dimai reported that nothing was visible at the location of the supernova
in J. Wray's Color Atlas of Galaxies.

W. Li, University of California at Berkeley, reported that the supernova was
unfiltered CCD magnitude 15.5 on an image taken with the Katzman Automatic
Imaging Telescope (KAIT) on Dec. 18.5 UT.  Nothing was visible at the position
of the supernova down to magnitude 19.0 on a KAIT image of NGC 4303 taken on
Nov. 26.5 UT.

A precise position of SN 1999gn, which is 31.7" east and 39.8" south of the
nucleus of the galaxy, was measured by Li as:

     R.A. = 12h 21m 57.02s   Decl.= +04 degrees 27' 45.6" (Equinox 2000.0)

Accompanying is a chart prepared by K. Malatesta, AAVSO Headquarters, using
an image from the Digitized Sky Survey and comparison star magnitudes from the
Hipparcos and Tycho Catalogues and from R. Buta (Thompson and Bryan, The
Supernova Search Charts and Manual).  Please use this chart to observe
1216+05 SN 1999GN and report your observations to AAVSO Headquarters so we
may inform the astronomical community.  Please be sure to indicate which
comparison stars you used.

Congratulations to Alessandro Dimai on his discovery!


Electronic copies of the SN 1999gn chart mentioned in this Alert Notice are
available through our web site at the following address:


The chart may also be obtained directly from our FTP site:

       ftp.aavso.org (, in /alerts/alert268

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
to observations@aavso.org.

Many thanks for your valuable astronomical contributions and your efforts.

Good observing!

Janet A. Mattei

Elizabeth O. Waagen
Senior Technical Assistant