We propose to have simultaneously X-ray and optical observations of the two X-ray binaries 4U 1254-69 4U 1916-053 using the rare combination of RXTE and UltraCam on the VLT. Both sources are at high inclination, giving rise to dips or eclipses in their lightcurves caused by obscuration by the outer edge of the accretion disk or the companion. Deriving high time resolution lightcurves in X-rays and optical during a dip or eclips is the only way that allows us to locate different emitting regions and thereby constrain the geometry of the accretion disk in X-ray binaries.