Electronic Telegram No. 5506 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 (10430) MARTSCHMIDT R. Nolthenius, Department of Astronomy, Cabrillo College, Aptos, CA, USA; J.-F. Gout, Tree Gate Farm Observatory, Starkville, MS, USA; and M. Conjat, Observatoire de la Cote d'Azur, report the discovery of minor planet (10430) as a binary asteroid. On 2024 Nov. 17.20 UT, the minor planet (magnitude V = 17.0) occulted the Tycho catalogue star 1814-01633-1 across a narrow path that included Cabrillo College Observatory, and co-author Nolthenius recorded the event with a Watec 910hx PAL video camera at f/3.3 on a 0.20-m Schmidt-Cassegrain telescope at single-frame integrations of 1/25 s. Data were reduced using PyMovie and PyOTE software to obtain light curves, contact timings, and the 2-sigma confidence limits quoted herein. The recording showed a deep first occultation of duration 0.41 +/- 0.026 s, followed by a gap of 0.62 s and a second deep occultation of duration 0.33 +/- 0.024 s. Both occultations reached sky level (i.e., limiting magnitude). Eight comparison stars of known B and V magnitudes were put on the Watec 910hx instrumental system (hereafter W magnitudes) using transformation coefficients posted at website URL http://www.hristopavlov.net/videocameras/ H. Pavlov of IOTA. The target star was measured at W = 11.31; comparison stars of W magnitudes 12.05, 12.09, 12.29, 12.58, 12.74, and 12.76 clearly showed on a 2.6-s stack of video frames to all be above the flat bottoms of both occultations, and above the observational limiting magnitude (measured to be W = 13.0, or 1.7 mag fainter than the target star). The depths for both occultations thus exceeded the 0.75-magnitude criterion for ruling out a binary-star explanation. The total duration from first to final contacts of 1.36 s (15.1 km) [vs. the 0.52 s maximum predicted at the Occultwatcher-Cloud website (Pavlov, cf. URL http://www.occultwatcher.net/desktop), which lists the diameter as 5.7 km (assuming a single object)] makes a graze on a peanut- shaped-asteroid explanation highly unlikely; such a graze would require a geometric aspect ratio above 3:1, given nominal albedo (which is more extreme than for any measured solar-system object). The chord lengths for the two occultations were 4.6 +/- 0.3 and 3.7 +/- 0.3 km, respectively. Minor planet (10430) was not found to be presently listed in the Johnstone archive of binary asteroids (https://johnstonsarchive.net/astro/asteroidmoons.html) or the GAIAMOONS list of potential binaries (https://www.oca.eu/fr/gaiamoons). Following co-author Nolthenius' announcement on the International Occultation Timing Association (IOTA) message board, photometry was obtained by co-authors Gout and Conjat over five nights. Gout observed from 2024 Nov. 20.23 to Nov. 22.43, and his photometry revealed a near-constant brightness at Sloan R magnitude 16.5, interrupted by a sharp 2.0-hour apparent eclipse of depth 0.75 magnitude on three successive nights. Spacing between eclipses was 23.19 +/- 0.02 hr. Conjat acquired 6.0 hr of photometry from 2024 Nov. 22.80 to Nov. 23.05, using a 0.4-m f/5 telescope at Nice Observatory, showing near-constant brightness with a 2.0-hr eclipse closely halfway between Gout's eclipses, of depth 0.52 mag in Gout's instrumental system. This corresponds to a lower limit of 0.78 on the size ratio of the two components. The combined light curve is of Algol type and thus strengthens the conclusion of (10430) as a binary asteroid. The shallower eclipse bottom of Conjat's data (vs. Gout's deeper sharp-bottomed eclipses) argues for Conjat having observed a secondary eclipse in a roughly circular orbital system with an orbital period of 23.19 +/- 0.02 hr and with roughly equal-sized components whose orbital plane closely aligned with the direction to the earth in mid-November 2024. The sky-plane component separation at the occultation suggests a lower limit on the separation between the centers of the two bodies of 10.7 km. Additional photometry is being done, some of which could be included in the authors' forthcoming Minor Planet Bulletin paper. The authors acknowledge help from P. Pravec, who reviewed the photometry of Gout and reviewed this text; from D. Gault using OCCULT4 software (written by D. Herald), who provided a tentative sky-plane view of the Nov. 17 occultation; and from D. Herald, who provided early comments on the writing of this submission. Details, light curves, and analysis are available at the following website URL: https://www.dr-ricknolthenius.com/events/20241117Martschmidt/index.html. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 February 21 (CBET 5506) Daniel W. E. Green