  The variational field equations of Brans-Dicke scalar-tensor theory of
gravitation are presented in a Riemannian and non-Riemannian setting in the
language of exterior differential forms over 4-dimensional spacetime. In Rosen
coordinates, the equations of motion of non-spinning test masses are considered
in a gravitational plane wave setup and detailed to interpret a scalar field
$\alpha$ as a candidate for dark energy for negative values of the Brans-Dicke
parameter $-3/2 < \omega < 0$ .
