VELOCE I. Cepheids High-precision RVs

Seventy-three spectroscopic binary Cepheids and 13 candidate binary Cepheids were identified using VELOCE data alone and are investigated in detail in a companion paper (VELOCE II). VELOCE DR1 provides several new insights into the pulsational variability of Cepheids. Chiefly among these are: a) the most detailed description of the Hertzsprung progression based on RVs to date, b) the identification of double-peaked bumps in the pulsation curve, and c) clear evidence that virtually all Cepheids feature spectroscopic variability signals that lead to modulated RV variability at the level of tens to hundreds of m/s that cannot be satisfactorily modeled using single-periodic Fourier series.

We identified 36 stars exhibiting such modulated variability, of which four also exhibit orbital motion. Linear radius variations depend strongly on pulsation period, and a steep increase in slope of the delta R/p vs log P-relation is found near 10 d. This effect, combined with significant RV amplitude differ- ences at fixed period challenge the existence of a tight relation between Baade-Wesselink projection factors and pulsation periods.

We investigated the accuracy of RV time-series measurements, v_gamma_, and RV amplitudes published by the ESA mission Gaia's third data release and determined an offset of 0.65+/-0.11km/s relative to VELOCE. Whenever possible, we recommend adopting a single set of template correlation parameters for distinct classes of large amplitude variable stars to avoid systematic offsets in v_gamma among stars belonging to the same class.

Peak-to-peak amplitudes of Gaia RVs exhibit significant (16%) dispersion. Potential differences of RV amplitudes require further inspection, notably in the context of projection factor calibration.