Radial velocity of V509 Cas. Upper panel: our radial velocity measurements based on TO and NOT spectra. Lower panel: radial velocity measurements from various authors – black crosses correspond to those from Harper (1923), green circles are for Lambert & Luck (1978), the black square is for Humphreys (1978) (exact observation date not given in the paper, only the year), and grey triangles are for Lambert et al. (1981) (S I, Ca II, and Si I lines).