  Based on RXTE/ASM and EXOSAT/ME data we studied X-ray variability of
persistent LMXBs in the 1e-8 - 1e-1 Hz frequency range, aiming to detect
features in their power density spectra (PDS) associated with the viscous time
scale of the accretion disk tvisc. As this is the longest intrinsic time scale
of the disk, the power density of its Mdot variations is expected to be
independent on the frequency at f<1/tvisc. Indeed, in the PDS of 11 sources out
of 12 we found very low frequency break, below which the spectra are nearly
flat. At higher frequencies they approximately follow the Pnu ~ nu^{-1.3} law.
  The break frequency correlates very well with the binary orbital frequency in
a broad range of binary periods Porb ~ 12 min - 33.5 days, in accord with
theoretical expectations for the viscous time scale of the disk. However, the
value of fbreak/forb is at least by an order of magnitude larger than predicted
by the standard disk theory. This suggests that a significant fraction of the
accretion Mdot occurs through the optically thin and hot coronal flow with the
aspect ratio of H/R>0.1 The predicted parameters of this flow, T ~ 1e4-1e6 K
and Ne ~ 1e12-1e15 cm^-3, are in qualitative agreement with recent Chandra and
XMM-Newton observations of complex absorption/emission features in the spectra
of LMXBs with high inclination angle.
  We find a clear dichotomy in the value of tvisc/Porb between wide and compact
systems, the compact systems having ~10 times shorter viscous time. The
boundary lies at the mass ratio q ~ 0.3, suggesting that this dichotomy is
caused by the excitation of the 3:1 inner Lindblad resonance in low-q LMXBs.
