The K2-OjOS Project: New and revisited planets and candidates in <i>K2</i> campaigns 5, 16, &amp; 18

We present the first results of K2-OjOS, a collaborative project between professional and amateur astronomers primarily aimed to detect, characterize, validate new extrasolar planets. For this work, 10 looked for planetary signals by visually inspecting 20 427 light curves K2 campaign 18 (C18). They found 42 planet candidates, which are detections 24 had been detected in overlapping C5 previous works. used archival photometric spectroscopic observations, as well high-spatial resolution images order carry out complete analysis candidates found, including homogeneous characterization host stars, transit modelling, search timing variations statistical validation. As result, we report four planets (K2-355 b, K2-356 K2-357 K2-358 b) 14 candidates. Besides, refine ephemeris previously published modelling C5, C16 (when available) C18 data jointly, largely improving period mid-transit time precision. Regarding individual systems, highlight b candidate EPIC 211537087.02 being near 2:1 commensurability, detection significant TTVs bright star K2-184 (V = 10.35), location K2-103 inside habitable zone according optimistic models, single known system K2-274, disposition reassignment K2-120 consider origin signal cannot be ascertained.