We thank Masaki Ando, Stanislav Babak, Christopher Berry, Emanuele Berti, Vitor Cardoso, Naoki
Seto, Kent Yagi, and the Living Reviews referees for useful comments on the manuscript; we thank
Christopher Moore for providing the image in Figure 2
. JG’s work is supported by the Royal Society. MV’s
work was carried out at the Jet Propulsion Laboratory under contract with the National Aeronautics and
Space Administration, with support from the LISA project and PCOS program. JB’s work was partly
supported by NASA grant 11-ATP11-046.