VELOCE modulation zoo. I.

The origin of the signal remains unknown. Here, we search for similar phenomena in spectroscopic observations of first-overtone classical Cepheids collected as part of the VELOCE project. We performed frequency analysis of several parameters derived from cross-correlation functions (CCFs), including radial velocity (RV), full width at half maximum (FWHM), bisector inverse span (BIS), and CCF depth (contrast).

Using standard prewhitening, we searched for additional low-amplitude signals. We identify the location of these stars in various sequences of the Petersen diagram. We detect additional signals in four first-overtone classical Cepheids: BG Cru, QZ Nor, V0391 Nor, and V0411 Lac.

We classified BG Cru, QZ Nor, and V0391 Nor as 0.61 stars based on period ratios. V0411 Lac, however, exhibits a ratio of 0.68 between the two modes, and the additional signal has a longer period. This kind of multiperiodicity remains unexplained.

VELOCE CCFs yield the first spectroscopic detections of non-radial pulsation modes in classical Cepheids. This opens an asteroseismic window for pursuing a more detailed understanding of these important stars. While the 0.61 signal of BG Cru, QZ Nor, V0391 Nor is understood to originate due to non-radial modes of moderate degrees, the 0.68 signal of V0411 Lac still lacks a physical explanation.