* Astronomy

Title: Three planets around HD 27894. A close-in pair with a 2:1 period ratio and an eccentric Jovian planet at 5.4 AU
Author: T. Trifonov, M. Kürster, M. Zechmeister, O. V. Zakhozhay, S. Reffert, M. H. Lee, F. Rodler, S. S. Vogt, S. S. Brems

Aims. Our new program with HARPS aims to detect mean motion resonant planetary systems around stars which were previously reported to have a single bona fide planet, often based only on sparse radial velocity data.
Methods. Archival and new HARPS radial velocities for the K2V star HD 27894 were combined and fitted with a three-planet self-consistent dynamical model. The best-fit orbit was tested for long-term stability.
Results. We find clear evidence that HD 27894 is hosting at least three massive planets. In addition to the already known Jovian planet with a period Pb approx 18 days we discover a Saturn-mass planet with Pc approx 36 days, likely in a 2:1 mean motion resonance with the first planet, and a cold massive planet (approx 5.3 MJup) with a period Pd approx 5170 days on a moderately eccentric orbit (ed = 0.39).
Conclusions. HD 27894 is hosting a massive, eccentric giant planet orbiting around a tightly packed inner pair of massive planets likely involved in an asymmetric 2:1 mean motion resonance. HD 27894 may be an important milestone for probing planetary formation and evolution scenarios.

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