  We present the results of the timing analysis of five Rossi X-ray Timing
Explorer observations of the Black Hole Candidate GRS 1915+105 between 1996
September and 1997 December. The aim was to investigate the possible presence
of a type-B quasi-periodic oscillation (QPO). Since in other systems this QPO
is found to appear during spectral transitions from Hard to Soft states, we
analyzed observations characterized by a fast and strong variability, in order
to have a large number of transitions. In GRS 1915+105, transitions occur on
very short time scales (~ sec): to single them out we averaged Power Density
Spectra following the regular path covered by the source on a 3D
Hardness-Hardness-Intensity Diagram. We identified both the type-C and the
type-B quasi-periodic oscillations (QPOs): this is the first detection of a
type-B QPO in GRS 1915+105. As the spectral transitions have been associated to
the emission and collimation of relativistic radio-jets, their presence in the
prototypical galactic jet source strengthens this connection.
