Left panel: normalized out-of-transit master spectrum of KELT-9 in the 387–687 nm optical region. The stellar A0 spectral type makes the spectrum dominated by strong Balmer lines, though we noticethe presence of numerous shallow metal lines, as shown in the inset. The Balmer lines (Hα, β, γ, δ, ɛ, ζ), that are covered by the HARPS-N spectrograph wavelength range, are identified by the vertical dashed lines. Right panel: normalized out-of-transit master cross-correlation function (CCF) obtained with a dedicated A0 mask. The averaged stellar line has a measured projected velocity broadening of v sin i_* = 112.6 km s⁻¹. The vertical dashed line represent the systemic velocity (−17.74 km s⁻¹).