  A four-parameter family of covariance functions for stationary Gaussian
processes is presented. We call it 2Dsys. It corresponds to the general
solution of an autonomous second-order linear stochastic differential equation,
thus arises naturally from modelling. It covers underdamped and overdamped
systems, so it is proposed to use this family when one wishes to decide if a
time-series corresponds to stochastically forced damped oscillations or a
stochastically forced overdamped system.
