0 v 1 2 8 M ar 2 00 3 Where is the second planet in the HD 160691 planetary system ?

The set of radial velocity measurements of the HD 160691 has been recently published by Jones et al. (2002b). It reveals a linear trend that indicates a presence of the second planet in this system. The preliminary double-Keplerian orbital fit to the observations, announced by the discovery team, describes a highly unstable, self-disrupting configuration. Because the observational window of the HD 160691 system is narrow, the orbital parameters of the hypothetical second companion are unconstrained. In this paper we try to find out whether a second giant planet can exist up to the distance of Jupiter and search for the dynamical constraints on its orbital parameters. Our analysis employs a combination of fitting algorithms and simultaneous examination of the dynamical stability of the obtained orbital fits. It reveals that if the semi-major axis of the second planet is smaller than ≃ 5.2 AU, the observations are consistent with quasi-periodic, regular motions of the system confined to the islands of various low-order mean motion resonances, e.g., 3:1, 7:2, 4:1, 5:1, or to their e-mail: [email protected] e-mail: [email protected] e-mail: [email protected]