Mean per epoch RV uncertainty of bona fide Cepheids with at least 9 observations as a function of the mean Gaia DR3 G-band magnitude, cf. Table A.1. Coralie uncertainties (green points) are estimated differently from Hermes uncertainties, cf. Sect. 2.3, and are limited by photon noise. Hermes uncertainties (red points) are overestimated for G ≲ 10 mag. The brightest star, Polaris, is missing due to a lack of Gaia photometry; the faintest target is GL Cyg. X Sgr is excluded from the plot due to significant line shape distortions (Mathias et al. 2006; Anderson 2013) that complicate uncertainty estimation.