Four new exoplanets and hints of additional substellar companions to exoplanet host stars

We present four new exoplanets: HIP 14810b and HIP 14810c, HD 154345b, and HD 187123c. The two planets orbiting HIP 14810, from the N2K project, have masses of 3.9 and 0.76 M_J. We have searched the radial velocity time series of 90 known exoplanet systems and found new residual trends due to additional, long period companions. Two stars known to host one exoplanet have sufficient curvature in the residuals to a one planet fit to constrain the minimum mass of the outer companion to be substellar: HD 68988c with 8 M _J < m sin i < 20 M_J and HD 187123c with 3 M _J < m sin i < 7 M_J, both with P > 8 yr. We have also searched the velocity residuals of known exoplanet systems for prospective low-amplitude exoplanets and present some candidates. We discuss techniques for constraining the mass and period of exoplanets in such cases, and for quantifying the significance of weak RV signals. We also present two substellar companions with incomplete orbits and periods longer than 8 yr: HD 24040b and HD 154345b with m sin i < 20 M_J and m sin i < 10 M_J, respectively.