The VELOCE modulation zoo

Context. Line splitting in spectral lines is observed in various types of stars due to phenomena such as shocks, spectroscopic binaries, magnetic fields, spots, and nonradial modes. In pulsating stars, line splitting is often attributed to pulsation-induced shocks. However, this is rarely observed in classical Cepheids, with only a few reports; these include X Sagittarii and BG Crucis, where it has been linked to atmospheric shocks. Aims. We aim to investigate line splitting in X Sgr and BG Cru using spectroscopic time series and search for similar phenomena in other classical Cepheids. Methods. High signal-to-noise cross-correlation-function (CCF) time series from the VELOcities of CEpheids (<ns0:monospace>VELOCE</ns0:monospace>) project are analyzed. This dataset spans several years, allowing us to study the periodicities and evolution of CCF features. For X Sgr and BG Cru, we performed a detailed analysis of the individual components of the split CCFs. Additionally, we searched for periodicities in CCF variations and examined other classical Cepheids for distortions resembling unresolved line splitting. Results. We confirm line splitting in X Sgr and BG Cru, trace the features over time, and uncover the periodicity behind them. Several other Cepheids also exhibit CCF humps, suggesting unresolved or marginally resolved line splitting. We discuss the incidence and characteristics of these stars. Conclusions. The periodicity of line splitting in X Sgr and BG Cru differs significantly from the dominant pulsation period, ruling out pulsation-induced shocks. The periodicities are too short for rotation-related phenomena, suggesting nonradial modes as the most likely explanation, though their exact nature remains unknown. We also identify humps in six additional stars, indicating an incidence rate of 3% in the <ns0:monospace>VELOCE</ns0:monospace> sample.