Low mass X-ray binaries are two gravitationally bound stars where the secondary is a normal solar type like star and the primary a compact object, i.e. a black hole or neutron star. Matter is transferred from the secondary, thereby forming an accretion disk around the compact object that gives rise to the X-rays. These systems are excellent laboratories to test several theories, including stellar evolution, theory of accretion and the theory of relativity. However, accurate measurements of the masses of the two components in these binaries are necessary to test these theories, and that has been complicated since the secondary is many magnitudes fainter than the accretion disk around the compact object. We propose a new technique to measure the masses more accurately than has been possible thus far. Recent observations have shown that a signature of the secondary is present in certain fluorescence lines, in particular the Bowen fluorescence lines around 4640 Angstroms. Furthermore, the observed optical emission is mainly due to reprocessing of the X-rays, and any variability observed in the X-rays should also be present in the optical, but delayed with a few seconds depending on the reprocessing time and distance of the optical site compared to that of the X-rays. Simultaneous high temporal X-ray and optical observations, either narrow band photometry or spectroscopy, should be able to measure the delay in the fluorescence lines coming from the donor, and thereby constrain the system parameters of low mass X-ray binaries. For the optical observations however, large 10 m class telescopes with high timing capabilities will be necessary, and these will become available early 2009 in the form of GranteCan. Here we ask for the grant to have minor adjustments to the instrumentation on this telescope that will be necessary to perform the science proposed.