Determination of RV zero-points as a function of time for different references. From top to bottom: Gorynya et al. (1998), the previously largest data set of classical Cepheid RV time series; Kienzle et al. (1999); Borgniet et al. (2019). Only stars with VELOCE observations are used to determine zero-points. Horizontal errorbars indicate the time span of the measurements. A solid red line shows an unweighted linear fit to investigate temporal trends; the dotted black line shows the median with its standard error as a grey shaded region.