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In an earlier campaign to characterize the mass of the transiting temperatesuper-Earth K2-18b with HARPS, a second, non-transiting planet was posited toexist in the system at $\sim 9$ days. Further radial velocity follow-up withthe CARMENES spectrograph visible channel revealed a much weaker signal at 9days which also appeared to vary chromatically and temporally leading to theconclusion that the origin of the 9 day signal was more likely to be related tostellar activity than to being planetary. Here we conduct a detailedre-analysis of all available RV time-series, including a set of 31 previouslyunpublished HARPS measurements, to investigate the effects of time-sampling andof simultaneous modelling of planetary + activity signals on the existence andorigin of the curious 9 day signal. We conclude that the 9 day signal is realand was initially seen to be suppressed in the CARMENES data due to a smallnumber of anomalous measurements, although the exact cause of these anomaliesremains unknown. Investigation of the signal's evolution in time, withwavelength, and detailed model comparison reveals that the 9 day signal is mostlikely planetary in nature. By this analysis, we reconcile the conflictingHARPS and CARMENES results and measure precise and self-consistent planetmasses of $m_{p,b} = 8.63 \pm 1.35$ and $m_{p,c}\sin{i_c}=5.62 \pm 0.84$M$_{\oplus}$. This work, along with the previously published RV papers on theK2-18 planetary system, highlight the importance of understanding one'stime-sampling and of simultaneous planet + stochastic activity modelling,particularly when searching for sub-Neptune-sized planets with radialvelocities.

Confirmation of the radial velocity super-Earth K2-18c with HARPS and CARMENES