Cross-correlation function and v_rad vs. template average line width σ_ℓ. Left: CCFs resulting from the cross-correlation of one observed δ Cep spectrum (see text) with the ALL template built with a variable σ_ℓ within the range 0.02–0.62 Å (y-axis). The CCFs tested at different σ_ℓ are shown in orange and the CCF corresponding to the default σ_ℓ ~ 0.43 used in this study (Table 2) is shown in red. The CCF corresponding to the same δ Cep spectrum and the G2 HARPS DRS template is showed as a dashed black curve and is shifted vertically for clarity. The four right plots show the behaviour of different CCF observables vs. σ_ℓ (from left to right and top to bottom; in red circles): (i) the CCF Q-factor averaged over the 103 δ Cep spectra; (ii) the same but for the averaged S/N_CCF criterion; (iii) the slope of the linear regression to the δ Cep BIS time series at a given σ_ℓ vs. the BIS time series at σ_ℓ ~ 0.43; and (iv) the same but for the RV_cc-c time series. On the four plots, the red diamond corresponds to the default σ_ℓ used in this study for the ALL template and the black square corresponds to the G2 HARPS DRS template.