  In contrast to Hermitian systems, eigenstates of non-Hermitian ones are in
general nonorthogonal. This feature is most pronounced at exceptional points
where several eigenstates are linearly dependent. In this work we show that
near this point a new effect takes place. It exhibits in energy increases in
the system when its parameters change periodically. This effect resembles
parametric resonance in a Hermitian system but there is a fundamental
difference. It comes from the unique properties of the exceptional point that
leads to parametric instability that occurs almost at any change in a
parameter, while in the case of Hermitian systems it is necessary to fulfill
resonance conditions. We illustrate this phenomenon by the case of two coupling
waveguides with gain and loss. This phenomenon opens a wide range of
applications in optics, plasmonics, and optoelectronics, where the loss is an
inevitable problem and plays a crucial role.
