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With about 2000 extrasolar planets confirmed, the results show that planetarysystems have a whole range of unexpected properties. We present a fullinvestigation of the HD219828 system, a bright metal-rich star for which a hotneptune has previously been detected. We used a set of HARPS, SOPHIE, andELODIE radial velocities to search for the existence of orbiting companions toHD219828. A dynamical analysis is also performed to study the stability of thesystem and to constrain the orbital parameters and planet masses. We announcethe discovery of a long period (P=13.1years) massive (msini=15.1MJup) companion(HD219828c) in a very eccentric orbit (e=0.81). The same data confirms theexistence of a hot-neptune, HD219828b, with a minimum mass of 21 MEarth and aperiod of 3.83days. The dynamical analysis shows that the system is stable. TheHD219828 system is extreme and unique in several aspects. First, among allknown exoplanet systems it presents an unusually high mass ratio. We also showthat systems like HD219828, with a hot neptune and a long-period massivecompanion are more frequent than similar systems with a hot jupiter instead.This suggests that the formation of hot neptunes follows a different path thanthe formation of their hot jovian counterparts. The high mass, long period, andeccentricity of HD219828c also make it a good target for Gaia astrometry aswell as a potential target for atmospheric characterisation, using directimaging or high-resolution spectroscopy. Astrometric observations will allow usto derive its real mass and orbital configuration. If a transit of HD219828b isdetected, we will be able to fully characterise the system, including therelative orbital inclinations. With a clearly known mass, HD219828c may becomea benchmark object for the range in between giant planets and brown dwarfs.

An extreme planetary system around HD219828. One long-period super Jupiter to a hot-neptune host star