Behaviour of the ambiguous M × ν_U(a) relations discussed in Sect. 3.1. 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. a) The M × ν_U(a) curves drawn for the Ep model. The two arrows indicate a region close to a = 1 where more than one resonant radii appear. b) An enlarged view of the curves drawn for the Ep model in the region close to a = 1. c) The M × ν_U(a) curves drawn for the RP1 model. Note the two branches of the curves and their full absence above a = 1.4. d) The M × ν_U(a) curves drawn for the RP2 model. Note the discontinuities in the displayed curves.