Constraints on Gravitational Dipole Radiation from Pulsars
  Recent suggestions for a modification of general relativity to provide an alternative approach to gravity in connection with the dark energy (matter) problem imply a long range vector component of the gravitational field. This could lead to emission of gravitational dipole emission from objects such as pulsars. Stringent observational limits on period changes of binary and millisecond pulsars and their consistency with general relativity impose severe limits on couplings of such forces. These bounds are tighter than those implied by lunar laser ranging experiments. 
C. Sivaram (Indian Institute of Astrophysics)