2010CBET.2574....1Z

W. Zheng, A. Romadan, and N. Whallon, University of Michigan; J. Vinko, University of Szeged; R. Quimby, California Institute of Technology; S. B. Pandey, F. Yuan, and C. Akerlof, University of Michigan; P. Pasque and M. Verkinderen, Cousino High School; and J. C. Wheeler and E. Chatzopoulos, University of Texas, on behalf of the ROTSE collaboration, report the discovery of an optical transient in M31, located at R.A. = 0:40:24.39, Decl. = +41:04:03.9 (equinox 2000.0; uncertainty < 1"), which is 26'.3 west and 12'.1 south from the core of M31, that was detected in unfiltered CCD images taken by the ROTSE-IIIb telescope at McDonald Observatory on Dec. 1.15 UT, both in co-added image and in four individual images at mag about 16.7. The object, designated 2010-12a, was not detected to a limiting magnitude of about 18.5 before and on Nov. 28. The quoted ROTSE magnitudes are calibrated relative to the USNO-B1.0 catalogue. A finding chart can be found at website URL http://www.rotse.net/rsvp/j004024.4+410404/ROTSE3-J004024.4+410404.jpg.