Coordinated thermal and optical observations of trans-Neptunian object (20000)Varuna from Sierra Nevada

We report on coordinated thermal and optical measurements of trans-Neptunian object (20 000)Varuna obtained in January-February 2002, respectively from the IRAM 30-m and IAA 1.5 m telescopes. The optical data show a lightcurve with a period of 3.176 +/- 0.010 hr, a mean V magnitude of 20.37 +/- 0.08 and a 0.42 +/- 0.01 magnitude amplitude. They also tentatively indicate that the lightcurve is asymmetric and double-peaked. The thermal observations indicate a 1.12 +/- 0.41 mJy flux, averaged over the object's rotation. Combining the two datasets, we infer that Varuna has a mean 1060(-220)(+180) km diameter and a mean 0.038(-0.010)(+0.022) V geometric albedo, in general agreement with an earlier determination using the same technique.