New limits on the strong equivalence principle from two long-period circular-orbit binary pulsars

Following a brief review of the principles of the strong equivalence principle (SEP) and tests for its violation in the strong and weak gravitational field regimes, we present preliminary results of new tests using two long-period binary pulsars: J0407+1607 and J2016+1947. PSR J0407+1607 is in a 669-day orbit around a $\gapp 0.2$ M$_{\odot}$ companion, while J2016+1947 is in a 635-day orbit around a $\gapp 0.3$ M$_{\odot}$ companion. The small eccentricities of both orbits ($e \sim 10^{-3}$) mean that these systems reduce previous limits on SEP violation by more than a factor of 4.