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Recent stellar occultations allowed accurate instantaneous size and apparentshape determinations of the large Kuiper belt object (50000) Quaoar anddetected two rings with spatially variable optical depth. In this paper wepresent new visible range light curve data of Quaoar from the Kepler/K2mission, and thermal light curves at 100 and 160\,$\mu$m obtained withHerschel/PACS. K2 data provide a single-peaked period of 8.88 h, very close tothe previously determined 8.84 h, and it favours an asymmetric double-peakedlight curve with 17.76 h period. We clearly detected a thermal light curve withrelative amplitudes of $\sim$10% both at 100 and 160 $\mu$m. A detailedthermophysical modeling of the system shows that the measurements can be bestfitted with a triaxial ellipsoid shape, with a volume-equivalent diameter of1090 km, and axis ratios of a/b = 1.19, and b/c = 1.16. This shape matches thepublished occultation shape, as well as visual and thermal light curve data.The radiometric size uncertainty remains relatively large ($\pm$40 km) as thering and satellite contributions to the system-integrated flux densities areunknown. In the less likely case of negligible ring/satellite contributions,Quaoar would have a size above 1100 km and a thermal inertia $\leq$ 10Jm$^{-2}$K$^{-1}$s$^{-1/2}$. A large and dark Weywot in combination with apossible ring contribution would lead to a size below 1080 km in combinationwith a thermal inertia $\gtrsim$ 10 Jm$^{-2}$K$^{-1}$s$^{-1/2}$, notably higherthan that of smaller Kuiper belt objects with similar albedo and colours. Wefind that Quaoar's density is in the range 1.67-1.77 g/cm$^3$ significantlylower than previous estimates. This density value fits very well to therelationship observed between the size and density of the largest Kuiper beltobjects.

On the visible and thermal light curve of the large Kuiper belt object (50000) Quaoar