Behaviour of the M × ν_U(a) functions in the case of the RP, Kep, TD, and WD models. The shaded rectangular area denotes the observational bounds on the quantity M × ν_U(a) determined for GRS 1915+105 (Table 1). This area also roughly indicates the upper bounds for the whole group of the three microquasars GRS 1915+105, XTE J1550−564, and GRO J1655−40. Note that the curve corresponding to the TD model joins those that correspond to the WD model. The coincidence follows from the model frequency relations and the requirement of the observed 3:2 frequency ratio.