Generalized Lomb-Scargle periodograms of RV data for GJ 251 for CARMENES, HIRES, and a combination of both. The stellar rotational period derived by photometry is plotted as the dashed red line, and the 1σ and 3σ uncertainties are highlighted in red. We also indicate the harmonic of the rotational period and its uncertainty. The green line marks the suspected planetary signal. The blue lines mark the periods of the published planetary candidates by Butler et al. (2017) at 1.7 d, and the significant HIRES signal around 604 d.