| Abstract
| - A lightcurve of Centaur 1999 UG 5 from R-band CCD images taken at the Calar Alto 1.52 m and La Palma 3.5 m TNG telescopes is presented. The lightcurve shows noticeable changes in brightness. Small activity outbursts do not appear to be the cause of the brightness changes because no coma was detected after coadding all the images. Thus, the changes in brightness are interpreted as being caused by rotational variability. A periodogram analysis of the lightcurve reveals significant peaks from 13.0 h to 13.8 h, with a confidence level exceeding 99.9% . The maximum spectral power corresponds to 13.25 h. The overall shape of the lightcurve can be explained by an irregular object rotating once per 26.5 h, but other possibilities exist. Assuming an ellipsoid shape for the rotating body, the $0.24\pm 0.02$ mag amplitude of the lightcurve implies a minimum axial ratio of 1.25. The mean absolute magnitude in V band was found to be $H_V=10.42\pm 0.02$ assuming a typical phase parameter $G=0.15$. This implies a diameter of 55.3 km for the object, provided that a typical albedo of 0.04 is assumed. The colours of 1999 UG 5 were found to be $B-V=0.95\pm 0.13$, $V-R=0.63\pm 0.06$, and $R-I=0.61\pm 0.09$.
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