Biaxial order in cholesteric liquid crystals: Phenomenological argument

On the basis of a phenomenological Landau free energy for liquid crystals which includes the Frank (elastic) free energy, and allows for nonvanishing biaxialities, it is shown that cholesteric liquid crystals are biaxial. The biaxialities are very small, of order (q0a)2 (q0 = pitch wave vector, a = a molecular length), in agreement with recent molecular model calculations.