  Systematic survey for multiperiodicity in the LMC Cepheids (Moskalik,
Kolaczkowski & Mizerski 2004, 2006) has led to discovery of several new forms
of pulsational behaviour. One of them is periodic amplitude and phase
modulation observed in many first/second overtone (FO/SO) double mode Cepheids.
In the current paper we present detailed discussion of this newly discovered
phenomenon, based on a combined OGLE+MACHO sample of double mode pulsators.
