We report about our study on unconventional density waves (UDW) (i.e. density waves with wavevector dependent gap) in quasi‐one dimensional systems. Due to the zero average of the gap over the Fermi surface, these systems are not characterized by the periodic modulation of either the charge or spin density, and are referred to as systems with hidden‐order parameter. The quantities signaling the phase transition are determined. Depending on the explicit wavevector dependence of the gap, optical absorption is possible at low frequencies. In the presence of magnetic field, the different phases exhibit distinct behaviour, and the nonlinear electric response is strongly influenced by the applied field. © 2003 American Institute of Physics