Non-radial Pulsation in the First Overtone Cepheids of the Magellanic Clouds

We have analysed photometric data for the first overtone Cepheids in the Magellanic Clouds from the OGLE collection. In more than 500 stars, we have detected additional variability with periods shorter than the first overtone period and period ratios in the (0.60, 0.65) range. The sample includes double periodic stars detected previously by the OGLE team as well as new discoveries. In the Petersen diagram, these stars form three well-separated sequences. In some stars, we have found simultaneously two close periodicities corresponding to two sequences in the Petersen diagram. In a significant fraction of stars, we have detected the power excess at half the frequency of the additional variability. Interestingly, most of these stars form the middle sequence in the Petersen diagram.