Hertzsprung progression illustrated by the model RV curves of 208 fitted Cepheids (11 low-amplitude stars were removed for clarity). Cepheids are offset in the vertical direction according to their log P_puls. Errorbars near the top show the constant velocity scale. Vertical dotted lines indicate ϕ = 0, the phase of minimum radius. Left panel, left column: Cepheids classified as first overtone pulsators in Gaia DR3. Left panel, right column: Cepheids classified as fundamental mode pulsators in Gaia DR3. Outliers with particularly small amplitudes are not shown for clarity (cf. Sect. 4.4). The shortest-period FU Cepheids is BP Cir. Right panels: close up view in the period range 9–18 days. Stars with double-peaked bump features are shown in blue on the right, stars with single-peaked bumps on the left in black. Names are color coded accordingly. Outliers Z Lac, CH Cas, and Y Oph are not shown for clarity. The double-peaked bump appears in 31/61 ≈ 50% of VELOCE Cepheids and may be a ubiquitous feature of Cepheid RV curves that requires very high precision and extremely dense phase sampling for detection.