  We suggest that persistent low-frequency quasi-periodic oscillations (QPOs)
detected in X-ray, ultraviolet, optical energy ranges the black hole (BH)
sources XTE J1118+480, GRO J1655-40 LMC X-1 at ~ 0.1 Hz, and QPOs in HZ Her/Her
X-1 at ~ 0.05 Hz and in Neutron Star (NS) binaries 4U 1323-62, 4U 1746-31 and
EXO 0748-76 at ~ 1 Hz are caused by the global disk oscillations in the
direction normal to the disk (normal mode). We argue that these disk
oscillations are a result of the gravitational interaction between the central
compact object and the disk. A small displacement of the disk from the
equatorial plane results in a linear gravitational restoring force opposite to
this displacement. Our analysis shows that the frequency of this mode is a
function of the mass of the central object and it also depends on the inner and
outer radii of the disk which in turn are related to the rotation period of the
binary system. We derive an analytical formula for the frequency of the normal
disk mode and show that these frequencies can be related to the persistent
lower QPO frequencies observed in the NS and BH sources. We offer a new
independent approach to the black hole mass determination by interpreting this
low QPO frequency as the global disk oscillation frequency. The implementation
of this method combined with the independent method which uses the X-ray energy
spectra (Shrader & Titarchuk 1999) results in stringent constraints for the
black hole masses.
