Echo-tomgraphy is an indirect imaging technique which uses multiwavelength observations of LMXBs to constrain the fundamental parameters of these binaries. In particular, it uses time delays between X-ray and UV/optical light-curves as a function of orbital phase in order to map the reprocessing sites in a binary. Since previous works probe that narrow components within the Bowen Blend (BB) emission line arise from reprocessing in the companion star we developped the idea of BB echo-tomography in order detect the echo signal from the donor at different orbital phases. The latter is strongly dependent on the inclination angle, binary separation and mass ratio and, therefore, can be used to set constraints on masses of the binary. Here, we present simultaneous high time resolution (1-10 Hz) X-ray and optical observations of the persistent LMXBs Sco X-1 and 4U 1636-536 (=V801 Ara). In the case of Sco X-1 we find that the Bowen/HeII emission lags the X-ray light-curves with a light travel time of 11- 16s which is consistent with reprocessing in the donor star. We also present the detection of three correlated X-ray/optical bursts in 4U 1636-536. Although this latter project is still in progress our preliminary results obtained by subtracting the Continuum light-curve from the Bowen/HeII data provide evidence of orbital phase dependent echoes from the companion star.