{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,4,27]],"date-time":"2026-04-27T23:54:50Z","timestamp":1777334090106,"version":"3.51.4"},"reference-count":34,"publisher":"EDP Sciences","license":[{"start":{"date-parts":[[2019,1,7]],"date-time":"2019-01-07T00:00:00Z","timestamp":1546819200000},"content-version":"vor","delay-in-days":6,"URL":"https:\/\/www.edpsciences.org\/en\/authors\/copyright-and-licensing"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["A&amp;A"],"accepted":{"date-parts":[[2018,10,9]]},"published-print":{"date-parts":[[2019,1]]},"abstract":"<jats:p>In an earlier campaign to characterize the mass of the transiting temperate super-Earth K2-18b with HARPS, a second, non-transiting planet was posited to exist in the system at ~9 days. Further radial velocity follow-up with the CARMENES spectrograph visible channel revealed a much weaker signal at 9 days, which also appeared to vary chromatically and temporally, leading to the conclusion that the origin of the 9-day signal was more likely related to stellar activity than to a planetary presence. Here we conduct a detailed reanalysis of all available RV time-series \u2013 including a set of 31 previously unpublished HARPS measurements \u2013 to investigate the effects of time-sampling and of simultaneous modelling of planetary plus activity signals on the existence and origin of the curious 9-day signal. We conclude that the 9-day signal is real and was initially seen to be suppressed in the CARMENES data due to a small number of anomalous measurements, although the exact cause of these anomalies remains unknown. Investigation of the signal\u2019s evolution in time with wavelength and detailed model comparison reveals that the 9-day signal is most likely planetary in nature. Using this analysis, we reconcile the conflicting HARPS and CARMENES results and measure precise and self-consistent planet masses of <jats:italic>m<\/jats:italic><jats:sub><jats:italic>p<\/jats:italic>,<jats:italic>b<\/jats:italic><\/jats:sub> = 8.63 \u00b1 1.35 and <jats:italic>m<\/jats:italic><jats:sub><jats:italic>p<\/jats:italic>,<jats:italic>c<\/jats:italic><\/jats:sub> sin<jats:italic>i<\/jats:italic><jats:sub><jats:italic>c<\/jats:italic><\/jats:sub> = 5.62 \u00b1 0.84 Earth masses. This work, along with the previously published RV papers on the K2-18 planetary system, highlights the importance of understanding the time-sampling and of modelling the simultaneous planet plus stochastic activity, particularly when searching for sub-Neptune-sized planets with radial velocities.<\/jats:p>","DOI":"10.1051\/0004-6361\/201833995","type":"journal-article","created":{"date-parts":[[2018,10,29]],"date-time":"2018-10-29T08:51:00Z","timestamp":1540803060000},"page":"A49","source":"Crossref","is-referenced-by-count":66,"title":["Confirmation of the radial velocity super-Earth K2-18c with HARPS and CARMENES"],"prefix":"10.1051","volume":"621","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-5383-9393","authenticated-orcid":false,"given":"R.","family":"Cloutier","sequence":"first","affiliation":[]},{"given":"N.","family":"Astudillo-Defru","sequence":"additional","affiliation":[]},{"given":"R.","family":"Doyon","sequence":"additional","affiliation":[]},{"given":"X.","family":"Bonfils","sequence":"additional","affiliation":[]},{"given":"J.-M.","family":"Almenara","sequence":"additional","affiliation":[]},{"given":"F.","family":"Bouchy","sequence":"additional","affiliation":[]},{"given":"X.","family":"Delfosse","sequence":"additional","affiliation":[]},{"given":"T.","family":"Forveille","sequence":"additional","affiliation":[]},{"given":"C.","family":"Lovis","sequence":"additional","affiliation":[]},{"given":"M.","family":"Mayor","sequence":"additional","affiliation":[]},{"given":"K.","family":"Menou","sequence":"additional","affiliation":[]},{"given":"F.","family":"Murgas","sequence":"additional","affiliation":[]},{"given":"F.","family":"Pepe","sequence":"additional","affiliation":[]},{"given":"N. 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