We discuss the constraints on the stronger sources of the cosmological gravitational waves background detectable by millisecond pulsars—upper bounds on the cosmic string parameter Gµ, on the anisotropy Δ of the QCD transition phase and, on the dimensionless gravity wave energy. We derive also upper limits on the string parameter from the future binary pulsar timings and confront them with those imposed by current elementary particle phenomenology. We wait for future observations and for new experiments which either detect or place significant constraints on the existence of strings and long wavelength gravity waves—millisecond pulsar plus binary pulsar and the COBE cosmic 3 K anisotropy experiment.
