Electronic Telegram No. 2105 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 (2131) MAYALL B. Warner, Palmer Divide Observatory; P. Pravec, P. Kusnirak, and K. Hornoch, Ondrejov Observatory; J. Vilagi, L. Kornos, S. Gajdos, P. Veres, and A. Galad, Modra Observatory; W. Cooney, J. Gross, and D. Terrell, Sonoita Research Observatory, Sonoita, AZ; and Yu. Krugly and V. Chiorny, Kharkiv Observatory, report that photometric observations obtained during Nov. 30-Dec. 16 reveal that minor planet (2131) is a binary system with an orbital period of 23.48 +/- 0.01 hr. The primary shows a period of 2.5678 +/- 0.0001 hr and has a lightcurve amplitude of 0.09 mag, suggesting a nearly spheroidal shape. The secondary rotates synchronously with the orbital motion, and its lightcurve component has an amplitude of 0.05 mag. Observed mutual-eclipse/occultation events indicate a secondary-to-primary mean-diameter ratio of 0.28 +/- 0.02. Observations in 2005 (Warner 2005, Minor Planet Bull. 32, 54-58) and 2007 (Warner et al. 2007, Minor Planet Bull. 34, 23) showed no indications of (2131) being binary. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2009 CBAT 2009 December 31 (CBET 2105) Daniel W. E. Green