  We propose Landau phenomenology for describing the phase transition from the
conventional nematic into the conical helical orientationally non-uniform
structure recently identified in liquid crystals formed by "banana"-shaped
molecules. The mean field predictions are mostly in agreement with experimental
data. Based on the analogy with de Gennes model, we argue that fluctuations of
the order parameter turn the transition to the first order phase transition
rather than continuous one predicted by the mean-field theory. This conclusion
is in agreement with experimental observations. We discuss the new Goldstone
mode to be observed in the low-temperature phase.
