Expected numbers of near-monochromatic GW signals generated more than a day before the merger that would be detectable in a year of LISA observation, as functions of the chirp mass ℳ₀ and redshift z (left panel) and of the chirp mass and symmetric mass ratio η (right panel). The merger rates assume the abundance of IMBHs described by Eq. (6) for BHs heavier than 10³ M_⊙ and a universal merger efficiency factor of p_BH = 0.17 inferred from IPTA data, as discussed in the text.