  We present a dimensional analysis of two characteristic time scales in the
boundary layer where the disk adjusts to the rotating neutron star (NS). The
boundary layer is treated as a transition region between the NS surface and the
first Keplerian orbit. The radial transport of the angular momentum in this
layer is controlled by a viscous force defined by the Reynolds number, which in
turn is related to the mass accretion rate. We show that the observed
low-Lorentzian frequency is associated with radial oscillations in the boundary
layer, where the observed break frequency is determined by the characteristic
diffusion time of the inward motion of the matter in the accretion flow.
Predictions of our model regarding relations between those two frequencies and
frequencies of kHz QPO's compare favorably with recent observations for the
source 4U 1728-34. This Letter contains a theoretical classification of kHz
QPO's in NS binaries and the related low frequency features. Thus, results
concerning the relationship of the low-Lorentzian frequency of viscous
oscillations and the break frequency are presented in the framework of our
model of kHz QPO's viewed as Keplerian oscillations in a rotating frame of
reference.
