Curves ν_U  ×  M(a) implied by the individual geodesic models. The light yellow rectangle indicates the observationaly determined interval of ν_U  ×  M for GRS 1915+105. The red dotted vertical line denotes a = 0.95. The red vertical arrow indicates the correction needed to match the upper limit to ν_U  ×  M with the RP model for this spin. The colour boxes are drawn for the mass and spectral spin estimates given by different authors for GRS 1915+105, GRO J1655-40 and XTE 1550-564 (see the main text for references). The dotted blue line indicates the lower observational limit to M  ×  ν_U that is roughly common to GRO J1655-405 and XTE 1550-564.