Supermassive black hole binaries (SMBHBs) are a natural consequence of galaxy mergers, and the most promising sources of low-frequency gravitational waves (GWs). The most massive SMBHBs (108–1010 Msol) emit nano-hertz GWs and are currently targeted by Pulsar Timing Arrays, like the North American Nanohertz Observatory for Gravitational waves (NANOGrav). We used the most recent NANOGrav dataset to place constraints on putative SMBHBs in ~200 massive galaxies within NANOGrav’s sensitivity volume (~500Mpc). For dozens of galaxies the limits are very informative and only very unequal binaries with mass ratio of a few percent are allowed. I will also discuss the first limit on the density of binaries delivered by major galaxy mergers based entirely on GW data.