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 .