Electronic Telegram No. 2086 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://www.cfa.harvard.edu/iau/cbat.html SUPERNOVA 2009mk IN PGC 474 R. Chornock and E. Berger, Harvard-Smithsonian Center for Astrophysics, report that a spectrum (range 470-890 nm) of 2009mk (cf. CBET 2080), taken on Dec. 19.16 UT with GMOS on the Gemini-South telescope, reveals that the object is a type-IIb supernova. Cross-correlation with a library of supernova spectra using the "SuperNova IDentification" code (SNID; Blondin and Tonry 2007, Ap.J. 666, 1024) indicates that 2009mk is most similar to SN 1996cb near maximum light. Strong H-alpha absorption is present, with the absorption minimum blueshifted by 13500 km/s, as well as additional absorptions from He I. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2009 CBAT 2009 December 20 (CBET 2086) Daniel W. E. Green