Visual magnitude for a system with the parameters of FO Aqr (see text), as a function of the mass-transfer rate from the secondary. The thick solid curve is the total luminosity; the thin red curve is the contribution from the disc, the blue curve is that of the secondary star, and the green curve is that of the hot spot. The white dwarf at a temperature of 10⁴ K corresponds to a magnitude m_V = 20.7. The dashed portion of these curves corresponds to unstable cases, in which a fraction of the disc sits on the intermediate branch of the S curve and is therefore not physical.