Electronic Telegram No. 2193 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 (8373) STEPHENGOULD Yurij N. Krugly, Institute of Astronomy, Kharkiv National University; R. Behrend, Geneva Observatory; P. Pravec, K. Hornoch, and P. Kusnirak, Ondrejov Observatory; A. Galad and P. Veres, Modra Observatory; R. Crippa, Fondazione Observatorio Astronomico di Tradate, Tradate, Italy; F. Manzini, Stazione Astronomica di Sozzago, Italy; M. Audejean, Chinon, France; L. Bernasconi, Les Engarouines, France; N. Gaftonyuk, Simeiz Observatory; B. D. Warner, Palmer Divide Observatory and Space Science Institute, Colorado Springs, CO, U.S.A.; and I. Molotov and L. Elenin, Keldysh Institute of Applied Mathematics, Moscow, report that photometric observations obtained during 2010 Jan. 16-Feb. 6 reveal that minor planet (8373) is a binary system with an orbital period of 34.15 +/- 0.1 hr. The primary showed a period of 4.4345 hr with a lightcurve amplitude of 0.33 to 0.39 mag. Earlier observations in this apparition taken on 2009 Nov. 30 and Dec. 22 showed a lower primary amplitude of 0.18 and 0.28 mag, respectively; the amplitude changes were likely caused by changes of the illumination and viewing aspect of the minor planet. Mutual eclipse/occultation events observed during Jan. 17-Feb. 1 indicate a lower limit on the secondary-to- primary mean-diameter ratio of 0.27. The events were more shallow on Feb. 4 and 6; a possible cause is that the system was moving away from the eclipse geometry. Measured color indices in the Johnson-Cousins system: B-R = 1.20 +/- 0.043, V-R = 0.435 +/- 0.041, R-I = 0.43 +/- 0.045. The calibrated data of Jan. 17 give the mean absolute magnitude as H_R = 14.2 +0.3/-0.4, assuming G = 0.2 +/- 0.2. Re-examination of 2004 observations by Warner (Minor Planet Bull. 31, 67) shows what appears to be an event that was overlooked in the original analysis. Since only one event was captured (2004 Jan. 9), it is not possible to determine the orbital period from the 2004 data. Images of some nights were co-added for up to 15 kiloseconds equivalent exposures (0.6-m class telescopes); there is no sign of cometary activity for this object. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2010 CBAT 2010 March 4 (CBET 2193) Daniel W. E. Green