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The Peculiar Photometric Properties of 2010 WG9: A Slowly-Rotating Trans-Neptunian Object from the Oort Cloud

We present long-term BVRI observations of 2010 WG9, an ~100-km diameter trans-Neptunian object (TNO) with an extremely high inclination of 70 deg discovered by the La Silla - QUEST southern sky survey. Most of the observations were obtained with ANDICAM on the SMARTS 1.3m at Cerro Tololo, Chile from Dec 2010 to Nov 2012. Additional observations were made with EFOSC2 on the 3.5-m NTT telescope of the European Southern Observatory at La Silla, Chile in Feb 2011. The observations reveal a sinusoidal light curve with amplitude 0.14 mag and period 5.4955 +/- 0.0025d, which is likely half the true rotation period. Such long rotation periods have previously been observed only for tidally-evolved binary TNOs, suggesting that 2010 WG9 may be such a system. We predict a nominal separation of at least 790 km, resolvable with HST and ground-based systems. We measure B-R = 1.318 +/- 0.029 and V-R = 0.520 +/- 0.018, consistent with the colors of modestly red Centaurs and Damocloids. At I-band wavelengths, we observe an unusually large variation of color with rotational phase, with R-I ranging from 0.394 +/- 0.025 to 0.571 +/- 0.044. We also measure an absolute R-band absolute magnitude of 7.93 +/- 0.05 and solar phase coefficient 0.049 +/- 0.019 mag/deg.