Instantaneous Fourier components of the disk’s surface density for the locally isothermal disk models of Sect. 3 with h = 0.02 (left panel) and h = 0.06 (right panel), respectively. Results are shown at t = 2550 yr (that is, after about 20 orbits of the secondary), and are obtained with q = 0.4. The azimuthal wavenumber for each Fourier component is indicated in the top-right corner of each panel.