Shell decomposition of HD 128621 spectra. The first shell is the one presenting the highest correlation with CCF RVs. The second shell is similar to a change of the line bisector and the time series of the related coefficient strongly correlated with the VSPAN of the CCF. Third shell, present mostly power at one-year and a half-year and is more related to instrumental systematics. The fourth component (fifth one according to the explained variance) seems once again to be related to stellar activity. The next components all possess a score lower than 80% and are therefore considered as less significant. After the decorrelation by the shell coefficients α_j (t), the planetary injected period at 54 days (vertical red line) becomes the dominant signal, with the other peaks at 47 and 63 days being the one-year aliases.