Transiting planets with orbital periods longer than 40 d are extremely rare among the 5000+ planets discovered so far. The lack of discoveries of this population poses a challenge to research into planetary demographics, formation, and evolution. In this talk, we present the detection and characterization of HD88986b, a potentially transiting sub-Neptune, possessing the longest orbital period among known transiting small planets (< 4 R$_{\oplus}$) with a precise mass measurement (\textbf{$\sigma_M/M$} > 25\%). Additionally, we identify the presence of a massive companion in a wider orbit around HD88986. To validate this discovery, we used a combination of more than 25 years of extensive radial velocity (RV) measurements (441 SOPHIE data points, 31 ELODIE data points, 34 HIRES data points), Gaia DR3 data, 21 years of photometric observations with the automatic photoelectric telescope (APT), 2 sectors of TESS data, and a 7-day observation of CHEOPS. Our analysis reveals that HD88986b, based on two potential single transits on sector 21 and sector 48, both consistent with the predicted transit time from the RV model, is potentially transiting. The joint analysis of RV and photometric data show that HD88986\,b has a radius of 2.49$\pm$\textbf{0.18} R$_{\oplus}$, a mass of 17.2\textbf{$^{\textbf{+4.0}}_{\textbf{-3.8}}$} M$_{\oplus}$, and it orbits every 146.05$^{+0.43}_{-0.40}$ d around a subgiant HD88986 which is one of the closest and brightest exoplanet host stars (G2V type, R=1.543 $\pm$ \textbf{0.065} R$_{\odot}$, V=$6.47\pm 0.01$ mag, distance=33.37$\pm$0.04 pc). The nature of the outer, massive companion is still to be confirmed; a joint analysis of RVs, Hipparcos, and Gaia astrometric data shows that with a 3$\sigma$ confidence interval, its semi-major axis is between 16.7 and 38.8 au and its mass is between 68 and 284 M$_{Jup}$. HD88986\,b's wide orbit suggests the planet did not undergo significant mass loss due to XUV radiation from its host star. Therefore, it probably maintained its primordial composition, allowing us to probe its formation scenario. Furthermore, the cold nature of HD88986\,b (460$\pm$8\, K), thanks to its long orbital period, will open up exciting opportunities for future studies of cold atmosphere composition characterization. Moreover, the existence of a massive companion alongside HD88986 b makes this system an interesting case study for understanding planetary formation and evolution.