Dispersion relations for the two-component stars plus gas system with finite thickness (Eq. (46)) are shown for Q_s = 1.5, Q_g = 1.3, and for different gas fractions: ϵ = 0.15 (top panel) and ϵ = 0.2 (bottom panel). The thickness of the stellar disc (β_s) is varied from 0.1 to 0.3, whereas the thickness of the gas disc (β_g) is kept fixed at 0.1 throughout all cases shown here. For comparison, the corresponding dispersion relation for the zero-thickness stars plus gas system is also shown (dashed black line).