We thank P. Freire for pointing out how useful PSR J0337+1715 could be for testing the SEP, L. Shao for providing an independent cross-check on the signature of Δ and K. Nordvedt for explaining why the signature of Δ differs from that in lunar laser ranging. A.M.A. is supported by a Netherlands Foundation for Scientific Research (NWO) Veni grant. N.V.G. is supported by NOVA. J.W.T.H. acknowledges funding from an NWO Vidi fellowship and from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Starting Grant agreement number 337062 (‘DRAGNET’). A.T.D. is the recipient of an Australian Research Council Future Fellowship (FT150100415). D.R.L. also received support from NSF award OIA-1458952. S.M.R. and I.H.S. are Senior Fellows of the Canadian Institute for Advanced Research. I.H.S. is also supported by an NSERC Discovery Grant. The NANOGrav project (involving D.L.K., D.R.L., R.S.L., S.M.R. and I.H.S.) receives support from National Science Foundation (NSF) Physics Frontiers Center award number 1430284. The National Radio Astronomy Observatory is a facility of the NSF operated under cooperative agreement by Associated Universities. The Arecibo Observatory is operated by SRI International under a cooperative agreement with the NSF (AST-1100968), and in alliance with Ana G. Mendez-Universidad Metropolitana and the Universities Space Research Association. The Green Bank Observatory is a facility of the NSF operated under cooperative agreement by Associated Universities. The WSRT is operated by ASTRON with contributions from NWO.

Reviewer information

Nature thanks P. Freire and C. Will for their contribution to the peer review of this work.