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We have measured the thermal flux emitted at 60-200 micron by the
Pluto-Charon system using ISOPHOT-C. This emission constrains the temperature
of Pluto's surface. Most interestingly, our data suggest the existence of
an infrared lightcurve, i.e. variations of the Pluto flux with rotational
phase. If confirmed, such variations would demonstrate that the surface
temperature of Pluto is spatially variable, which has very important
implications for the atmosphere/surface interactions and the atmospheric
dynamics. We need a few additional measurements to confirm completely
unambiguously that these variations are real. In addition, measurements
at 25 micron with PHT-P would provide further insight into the
temperature distribution at Pluto's surface.