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Context. The study of Quasi-Periodic Oscillations (QPO) at low and highfrequency in the variability of the high-energy emission from black-holebinaries and their physical interpretation in terms of signatures of GeneralRelativity in the strong-field regime. Aims. To understand the nature of the 67Hz QPOs observed in the X-ray emission of the peculiar black-hole binary GRS1915+105 within the general classification of QPO and to determine the spin ofthe black hole in the system by applying the Relativistic Precession Model(RPM). Methods. Within the RPM, the only relativistic frequency that is stablein time over a large range of accretion rates and can be as low as 67 Hz (for ablack-hole mass as measured dynamically) is the Lense-Thirring frequency at theInnermost Stable Circular Orbit (ISCO). In the application of the model, thiscorresponds to type-C QPOs. Under this assumption, it is possible to measurethe spin of the black hole. We re-analysed a large number of RossiXTEobservations to check whether other timing features confirm this hypothesis.Results. The identification of the 67 Hz QPO as the Lense-Thirring frequency atISCO yields a value of 0.706 +/- 0.034 for the black hole spin. With this spin,the only two QPO detections at higher frequencies available in the literatureare consistent with being orbital frequencies at a radius outside ISCO. Thehigh-frequency bumps often observed at frequencies between 10 and 200 Hz followthe correlation expected for orbital and periastron-precession frequencies ateven larger radii.

Rethinking the 67 Hz QPO in GRS 1915+105: type-C QPOs at the innermost stable circular orbit