Based on RXTE/ASM and EXOSAT/ME data we studied X-ray variability of persistent LMXBs in the \sim 10 ^ { -8 } -10 ^ { -1 } Hz frequency range , aiming to detect features in their power density spectra ( PDS ) associated with the viscous time scale of the accretion disk t _ { visc } .
As this is the longest intrinsic time scale of the disk , the power density of its \dot { M } variations is expected to be independent on the frequency at f \la 1 / t _ { visc } .
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 P _ { \nu } \propto \nu ^ { -1.3 } law .

The break frequency correlates very well with the binary orbital frequency in a broad range of binary periods P _ { orb } \sim 12 { ~ { } min } -33.5 { ~ { } days } , in accord with theoretical expectations for the viscous time scale of the disk .
However , the value of f _ { break } / f _ { orb } 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 \dot { M } occurs through the optically thin and hot coronal flow with the aspect ratio of H / R \ga 0.1 .
The predicted parameters of this flow , T \sim 10 ^ { 4 } -10 ^ { 6 } K and n _ { e } \sim 10 ^ { 12 } -10 ^ { 15 } 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 t _ { visc } / P _ { orb } between wide and compact systems , the compact systems having \sim 10 times shorter viscous time .
The boundary lies at the mass ration q \approx 0.3 , suggesting that this dichotomy is caused by the excitation of the 3:1 inner Lindblad resonance in low- q LMXBs .