  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.
