Optical magnitude for a system with the parameters of FO Aqr (see text) and μ = 1.5 × 10³³ G cm³, as a function of the mass-transfer rate. The thick solid line is the total optical luminosity; the thin dashed curve represents the disc contribution and the upper and lower thin solid curves represent the secondary and hot spot contributions, respectively. The red portion of the curve corresponds to discless accretion. The grey area corresponds to the optical luminosity variations between the normal- and low states.