Alias analysis of long-term detrended signals near 13.7 d in: (a,b) LBL RV, (c,d) CCF FWHM, (e,f) S-index, (g,h) Hα, (i,j) Na I. The left column shows aliases of the 13.7 d signal, found in RV and attributed to GJ 3998 c. The right column shows aliases of half the P_rot peak found in the raw-data GLSP, at 15.4 d. (k) The WF reveals five prominent peaks at: 120.6 d (upward teal triangle), 194.0 d (pink hexagon), 339.5 d (blue circle), 369.6 d (green square) and 410.9 d (downward red triangle). Relevant aliases in data periodograms are plotted with dashed lines of a corresponding colour and marker. There are other strong peaks in the WF that do not create aliases in the specified period range (hollow grey circles).