Electronic Telegram No. 509 Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A. IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions) CBAT@CFA.HARVARD.EDU (science) URL http://cfa-www.harvard.edu/iau/cbat.html SUPERNOVA 2006cg Further to IAUC 8622, R. Quimby and P. Mondol, University of Texas, report the discovery of an apparent supernova in unfiltered CCD images taken under poor conditions on May 6.23 (at mag about 16.6), 12.25 (mag about 16.7), and 14.23 UT (mag about 16.5) using the 0.45-m ROTSE-IIIb telescope at the McDonald Observatory. SN 2006cg is located at R.A. = 13h05m02s.32, Decl. = +28o44'24".0 (equinox 2000.0; estimated uncertainty 1".5 in each coordinate), which is 2" west and 3" north of the center of the apparent host galaxy (catalogued redshift of z = 0.028 from Rines et al. 2003, A.J. 126, 2152; via NED). SN 2006cg was found by subtracting a co-addition of images taken between 2004 Dec. 15 and 2005 June 29 (limiting mag about 19.4) from the 2006 May images. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2006 CBAT 2006 May 14 (CBET 509) Daniel W. E. Green