Electronic Telegram No. 4321 Central Bureau for Astronomical Telegrams Mailing address: Hoffman Lab 209; Harvard University; 20 Oxford St.; Cambridge, MA 02138; U.S.A. e-mail: cbatiau@eps.harvard.edu (alternate cbat@iau.org) URL http://www.cbat.eps.harvard.edu/index.html Prepared using the Tamkin Foundation Computer Network COMET C/2016 R3 (BORISOV) Gennady Borisov reports his discovery of a diffuse comet with a coma diameter of 16" on three unfiltered 70-s exposures obtained on Sept. 11.06 UT through a 0.3-m f/1.5 Genon astrograph at the MARGO observatory (near Nauchnij, Crimea). The total red magnitude of the comet was 16.0 as measured in a circular aperture of size 0'.5. The discovery observations are tabulted below, along with follow-up observations from the following night. 2016 UT R.A. (2000) Decl. Mag. Observer Sept.11.06676 9 45 21.07 +27 53 36.3 16.0 Borisov 11.07516 9 45 24.56 +27 53 31.2 16.0 " 11.08356 9 45 27.74 +27 53 27.0 16.0 " 12.06029 9 52 16.14 +27 42 51.3 16.0 " 12.06456 9 52 18.12 +27 42 51.0 16.0 " 12.07168 9 52 21.11 +27 42 45.7 16.0 " After the object was posted on the Minor Planet Center's PCCP webpage, other CCD astrometrists have confirmed the object's cometary appearance. H. Sato, Tokyo, Japan, stacked twelve 20-s exposures taken on Sept. 12.5 UT with an iTelescope 0.61-m f/6.5 astrograph (+ luminance filter) at the Sierra Remote Observatory, Auberry, CA, USA, to find a strongly condensed coma 8" in diameter with a hint of tail was 20" long toward p.a. 260 degrees; the w-band magnitude was 17.9 as measured within a circular aperture of radius 5".0. P. Birtwhistle, Great Shefford, Berkshire, England, writes that images obtained with a 0.40-m f/6 Schmidt-Cassegrain reflector in twilight (with clouds interfering) on Sept. 13.2 show the comet to be very diffuse 15" coma with a poorly defined central condensation; the magnitude was measured to be 17.5 in a photometric aperture with 12".8 radius. The comet appeared similarly to Birtwhistle on Sept. 14.2 in worse observing conditions, and the magnitude was given as 17.0; on Sept. 15.2, the very diffuse coma was as large as 30" in diameter with no tail, and the magnitude given as 17.7. J.-F. Soulier obtained unfiltered images with a 0.30-m f/3.8 Newtonian reflector at Maisoncelles, France, on Sept. 14.14-14.16 that show a diffuse 35" coma with no tail; the red magnitude was 18.0 in a photometric aperture of radius 6".5. A. Hale, Cloudcroft, NM, USA, reports that he detected the comet visually at altitude 14 degrees in zodiacal light and early twilight with a 0.41-m reflector on Sept. 14.48, finding total magnitude about 12.9 with a 1'.4 coma. The available astrometry, the following preliminary parabolic orbital elements by G. V. Williams, and an ephemeris appear on MPEC 2016-S03. T = 2016 Oct. 10.9706 TT Peri. = 117.8863 Node = 78.6961 2000.0 q = 0.448367 AU Incl. = 53.0108 M. Meyer, Limburg, Germany, has suggested similarity of the orbital elements of this comet with those of comet C/1915 R1 (Mellish), for which a parabolic orbit was published by Einaarson and Alter (1915, Lick Obs. Bull. 8, 151) based on only three observations (which was then compared to other available observations). Williams notes that the available observations for C/2016 R3 are consistent with intermediate-period orbits with periods as short as 50 years, possibly even 30 years. A new orbit by Williams for C/1915 R1, based on seven observations spanning 1915 Sept. 19-23 is given below for comparison: T = 1915 Oct. 13.6001 TT Peri. = 116.1746 Node = 77.5290 2000.0 q = 0.464924 AU Incl. = 52.3278 Attempts by Williams and S. Nakano to definitively link the 1915 and 2016 apparitions (for zero, 1, 2, and 3 potential missed returns between 1915 and 2016) have not been successful. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2016 CBAT 2016 September 16 (CBET 4321) Daniel W. E. Green