We revisited 32 planetary systems around evolved stars observed within the framework of the Okayama Planet Search Program (OPSP) and its collaborative framework of the East Asian Planet Search Program Network (EAPS-Net). In this study, we aim to hunt for extra companions among the planet-harboring stars so as to study the properties of stars and giant planets in multiple-planet systems. With our latest radial velocities obtained from Okayama Astrophysical Observatory (OAO), we confirmed an additional giant planet in the wide orbit of 75 Cet system. We also presented planetary systems showing interesting radial velocity variations, including eps Tau, 11 Com, 24 Boo, 41 Lyn, 14 And, HD 32518, and ome Ser. In addition, we performed joint analysis by combining radial velocities with astrometry to constrain the orbital configuration of the HD 5608, HD 14067, HD 120084, and HD 175679 systems, whose host central stars illustrated significant astrometric accelerations. As for other systems, we simply refined their orbital parameters. We noted that the OPSP planet-harboring stars are averagely metal-poor, which could be induced by B-V color cut-off.
By investigating planet-harboring evolved stars, we suggest that metallicity tends positively correlate with multiplicity and the total planet mass of the system.