  In X-ray binaries, several percent of the compact object luminosity is
intercepted by the surface of the normal companion and re-radiated through
Compton reflection and the K-fluorescence. This reflected emission follows the
variability of the compact object with a delay approximately equal to the
orbital radius divided by the speed of light. This provides the possibility of
measuring the orbital radius and thus substantially refining the compact object
mass determination compared to using optical data alone. We demonstrate that it
may be feasible to measure the time delay between the direct and reflected
emission using cross-correlation of the light curves observed near the Kalpha
line and above the K-edge of neutral iron. In the case of Cyg X-1, the time
delay measurement is feasible with a 300--1000 ksec observation by a telescope
with a 1000 cm^2 effective area near 6.4 keV and with a ~5eV energy resolution.
With longer exposures, it may be possible to obtain mass constraints even if an
X-ray source in the binary system lacks an optical counterpart.
