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In this study, we revisit 32 planetary systems around evolved stars observedwithin the framework of the Okayama Planet Search Program and its collaborativeframework of the EAPS-Net to search for additional companions and investigatethe properties of stars and giant planets in multiple-planet systems. With ourlatest radial velocities obtained from Okayama Astrophysical Observatory (OAO),we confirm an additional giant planet in the wide orbit of 75 Cet system($P_{\rm{c}} = 2051.62_{-40.47}^{+45.98}\ \rm{d}$, $M_{\rm{c}}\sini=0.912_{-0.090}^{+0.088}\ M_{\rm{J}}$, and$a_{\rm{c}}=3.929_{-0.058}^{+0.052}\ \rm{au}$), along with five starsexhibiting long-term radial velocity accelerations, which indicates massivecompanions in the wide orbits. We have also found that the radial velocityvariations of several planet-harboring stars may indicate additional planetcandidates, stellar activities, or other understudied sources. These starsinclude $\epsilon$ Tau, 11 Com, 24 Boo, 41 Lyn, 14 And, HD 32518, and $\omega$Ser. We further constrain the orbital configuration of the HD 5608, HD 14067,HD 120084, and HD 175679 systems by combining radial velocities withastrometry, as their host central stars exhibit significant astrometricaccelerations. For other systems, we simply refine their orbital parameters.Moreover, our study indicates that the OPSP planet-harboring stars are moremetal-poor compared to the currently known planet-harboring stars, and this islikely due to the $B-V$ color upper limit at 1.0 for star selection in thebeginning of the survey. Finally, by investigating the less-massive giantplanets ($< 5 M_{\rm{J}}$) around currently known planet-harboring evolvedstars, we have found that metallicity positively correlates with themultiplicity and total planet mass of the system, which can be evidence for thecore-accretion planet formation model.

Revisiting Planetary Systems in Okayama Planet Search Program: A new  long-period planet, RV astrometry joint analysis, and  multiplicity-metallicity trend around evolved stars