  Double-mode Cepheid behaviour may arise either from non-resonant or resonant
mode interaction. Inclusion of turbulent convection into pulsation codes by
Kollath et al. (1998) led to stable double-mode F+1O Cepheid pulsation.
However, our recent computations (Smolec & Moskalik 2008) indicate, that these
models resulted from incorrect neglect of negative buoyancy effects. Once these
effects are taken into account, only some resonant double-mode models could be
found. These models do not solve the puzzle of double-mode phenomenon, as they
are restricted to narrow parameter ranges. For majority of the observed
double-mode Cepheids non-resonant mechanism has to be operational.
