The first observed stellar occultations by the irregular satellite (Saturn IX) Phoebe and improved rotational period

We report six stellar occultationsby ( SaturnIX) Phoebe, an irregular satellite of Saturn, obtained between mid-2017 and mid-2019. The 2017 July 06 event is the first stellar occultationby an irregular satellite ever observed. The occultationchords were compared to a 3D shape model of the satellite obtained from Cassini observations. The rotation periodavailable in the literature led to a sub- observerpoint at the moment of the observed occultationswhere the chords could not fit the 3D model. A procedure was developed to identify the correct sub- observer longitude. It allowed us to obtain the rotation periodwith improved precision over currently known value from literature. We show that the difference between the observedand the predicted sub- observer longitudesuggests two possible solutions for the rotation period. By comparing these values with recently observedrotational light curves and single-chord stellar occultations, we can identify the best solution for Phoebe's rotational periodas $9.27365 \pm 0.00002$ h. From the stellar occultations, we also obtained 6 geocentric astrometric positions in the ICRS as realised by the Gaia-DR2 with uncertainties at the 1-mas level.