Spectra of atomic ISM lines. Upper panels: observed spectra (black) of the interstellar K I line at λ_air = 7698.965 Å fitted with a two-Gaussian absorption model (red). Component numbers are indicated. The vertical dotted lines represent the laboratory wavelengths of the K I lines. The strong line at about λ_air = 9697.3 Å in panel d is a telluric O₂ line. Lower panels: observed spectra (black) of the Na I D₁ line at λ_air = 5895.924 Å (vertical dotted). For the cooler stars, HD 165784 and HD 111613, the interstellar Na I D₁ lines are blended with a stellar component. The third Na I component at ~5895.3 Å in panel h is very weak compared to the other two and barely present in K I, such that it is neglected for the RV analysis as little DIB absorption can be expected to originate from it. The position of the K I and Na I line in the stellar rest frame is indicated by a vertical arrow. Wavelengths are in the LSR.