  2007OR10 is currently the third largest known dwarf planet in the
transneptunian region, with an effective radiometric diameter of ~1535 km. It
has a slow rotation period of ~45 h that was suspected to be caused by tidal
interactions with a satellite undetected at that time. Here we report on the
discovery of a likely moon of 2007OR10, identified on archival Hubble Space
Telescope WFC3/UVIS system images. Although the satellite is detected at two
epochs, this does not allow an unambiguous determination of the orbit and the
orbital period. A feasible 1.5-5.8x10^21 kg estimate for the system mass leads
to a likely 35 to 100 d orbital period. The moon is about 4.2m fainter than
2007OR10 in HST images that corresponds to a diameter of 237 km assuming equal
albedos with the primary. Due to the relatively small size of the moon the
previous size and albedo estimates for the primary remains unchanged. With this
discovery all trans-Neptunian objects larger than 1000 km are now known to
harbour satellites, an important constraint for moon formation theories in the
young Solar system.
