I outline a new hypothetical approach issuing a second gravitational equation in the scope of a promising model tackling the gravitational wave problem. This wave equation for graviton is framed in the endeavour to bridge the puzzling missing link to allow for quantum scale physics in a unifying gravity theory, through a new coupling constant S: thus wave is regarded as a symmetry breaking of general covariance of field equations through contraction of Riemann tensor by a constant tensor. That also allows an inertial mass to be assigned to the graviton (OE-25 eV/c²). This extension of General Relativity stems from self-evident considerations on the differential conditions of compatibility involving the two fundamental tensors on the curvature of the Space-Time continuum. Some considerations about last detected events are broached on the gauging of S constant, bringing forth a value that differs of two orders of magnitude with respect to the fitting of known binary star systems, unless source parameters are revised.