Demonstration of a Doppler shift caused by changes in the radial velocity of 18 Peg: the black solid and red dashed lines are observed Uves spectra with R ≈ 55 000 taken about 72 days apart (MJD 53 152.42 and 53 224.31, respectively). Left: a clear wavelength shift is visible for the stellar Si ii lines whose asymmetric line profiles are due to slow stellar pulsations (see Sect. 3). Right: interstellar K i and telluric lines are shown to demonstrate that the barycentric correction was properly accounted for.