Electronic Telegram No. 5517 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 (313538) 2002 YB_12 P. Pravec, P. Kusnirak, K. Hornoch, P. Fatka, and H. Kucakova, Ondrejov Observatory, report that photometric observations taken with a 1.54-m telescope at the La Silla station of the European Southern Observatory during 2025 Jan. 2-Feb. 28 reveal that the Apollo-type minor planet (313538) is a binary system with an orbital period of 12.52 +/- 0.01 hr. The primary has a period of 2.6083 +/- 0.0005 hr and a lightcurve amplitude of 0.08 magnitude at solar phases 11-12 degrees, suggesting a nearly spheroidal shape. Mutual eclipse/occultation events that are 0.16 magnitude deep indicate a secondary- to-primary mean-diameter ratio of 0.40 +/- 0.02. The secondary appears synchronous and has a light-curve amplitude (in the combined primary-plus- secondary light curve) of 0.08 mag, suggesting an elongated shape with an equatorial-axis ratio of 1.56 +/- 0.13. The system's mean absolute magnitude is H = 18.46 +/- 0.09, assuming a phase-relation slope parameter G = 0.24 +/- 0.11. The color index in the Johnson-Cousins photometric system is (V - R) = 0.439 +/- 0.012. These observations were taken in the framework of their NEOSource project, supported by the "Praemium Academiae" grant from the Academy of Sciences of the Czech Republic. Follow-up observations (e.g., photometric or spectroscopic) taken during the rest of the current apparition of the minor planet are encouraged, to improve the characterization of the binary system. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 February 28 (CBET 5517) Daniel W. E. Green