Electronic Telegram No. 2305 Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION CBAT Director: Daniel W. E. Green; Room 209; Dept. of Earth and Planetary Sciences; Harvard University; 20 Oxford St.; Cambridge, MA 02138; U.S.A. e-mail: cbat@iau.org; cbatiau@eps.harvard.edu URL http://www.cfa.harvard.edu/iau/cbat.html APPARENT NOVA IN M31: M31N 2010-05a Kamil Hornoch, Ondrejov Observatory, reports the discovery of an apparent nova in the galaxy M31 by M. Wolf, P. Hornochova, and himself on co-added 990-s and co-added 900-s R-band CCD frames taken with the 0.65-m telescope at Ondrejov during May 28.035-28.062 UT; the new object is visible on single images used for the co-added images, but is not present on numerous archival images taken at Lelekovice and Ondrejov back to 2002. M31N 2010-05a is located at R.A. = 0h42m35s.88 +/- 0s.03, Decl. = +41o16'37".7 +/- 0".5 (equinox 2000.0), which is 95" west and 29" north of the center of M31. Available R-band magnitudes for M31N 2010-05a from various CCD images: 2010 Jan. 31.838, [21.2 (P. Kubanek, N. Morales, J. L. Ortiz, J. Gorosabel, and M. Jelinek, 1.23-m telescope at Calar Alto); Feb. 25.745, [20.1 (Hornoch, 0.65-m telescope at Ondrejov); Mar. 17.763, [19.9 (Hornoch, Wolf. and P. Zasche, 0.65-m telescope at Ondrejov); May 28.035, 17.6 +/- 0.15 (Hornoch, Wolf, and Hornochova, 0.65-m telescope at Ondrejov); 28.049, 17.4 +/- 0.2 (Hornoch, Wolf and Hornochova); 28.062, 17.4 +/- 0.2 (Hornoch, Wolf and Hornochova); June 4.135, 17.11 +/- 0.07 (Morales, Ortiz, Gorosabel, Kubanek, and Jelinek, 1.23-m telescope at Calar Alto). Koichi Nishiyama, Kurume, Japan; and Fujio Kabashima, Miyaki, Japan, report their independent discovery of M31N 2010-05a at mag 17.4 on seven 60-s unfiltered CCD frames (limiting magnitude 18.9), taken around June 3.784 UT using a Meade 200R 0.40-m f/9.8 reflector (+ SBIG STL1001E camera). They measure position end figures 35s.90, 37".9, and offset 95" west and 9" north of the center of the galaxy M31. Nothing is visible at this position on their frames taken on May 20.784 UT (limiting magnitude 19.0) and May 31.772 (limiting magnitude 18.8) or on Digitized Sky Survey red and infrared plates. They note that a star is present within an arcsec of this position in Massey's M31 catalogue (http://www.lowell.edu/users/massey/lgsurvey.html), with position end figures 35s.92, 37".2 and magnitudes V = 21.1, B = 22.1, R = 20.3. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2010 CBAT 2010 June 4 (CBET 2305) Daniel W. E. Green