Unveiling hidden pulsation modes in Galactic first overtone Cepheids with TESS

Classical Cepheids pulsating in the first radial overtone may exhibit low-amplitude additional periodicities at the millimagnitude level. These signals form distinct sequences in the Petersen diagram near the 0.61 and 0.68 period ratios, similar to those found in RR Lyrae stars. The origin of these signals remains an open question; however, theoretical models involving non-radial modes offer a promising explanation for the former group. While extensive OGLE surveys in the Magellanic Clouds have revealed such low-amplitude signals in overtone Cepheids, the Galactic sample remains much less explored. In this talk, I highlight the potential of TESS full-frame image data to detect these signals in the first overtone Cepheids of the Milky Way. Using Fourier analysis techniques, we identify over a hundred stars that exhibit additional periodicities. In a limited number of cases, temporal variations in these signals can also be examined, posing further challenges to existing theoretical models.