X-ray binaries (XRBs) are arguably the best sources for studying accretion and jets, because they traverse various accretion states with greatly differing jet strength, over timescales of the order of months, easily amenable to observation. However, despite the rapid increase in our phenomenological knowlege of jets, we still lack a fundamental understanding of how jets are powered and collimated, or what are the bulk and internal properties of jets. Furthermore, the interplay between the radiative inflow via an accretion discand the launching of jets is still not entirely clear. Rapid (sub-second) multi-wavelength monitoring of black hole (BH)and neutron star (NS) XRBs have opened up a new window into accretion and jet studies. This is because fast optical/near-IR timing is able to probe plasma flowsmuch closer to the jet base than ispossible at radio wavelenghts. Our previous results suggest a causal connection with anti-correlated optical/X-ray behaviour observed on timescales of seconds, interpreted as arising from a hot synchrotron flow.More, recently our detection of a 100-ms optical/X-ray delay associated with the inner jet in three BH-XRBs, points to an unknown mechanism governing the scale of plasma acceleration along the jet, a mechanism that is likely independent of BH spin and internal shock characteristics. Multi-wavelength variability data, combined with timing analysis techniques, provides information about the relationship between different physical components in accreting XRBs. It is a powerful tool to study accretion disc-jet coupling, constraining jet launching mechanisms, and the posible interaction between the magnetic field of the NS and the accretion disc. However, with just a handful of observations, we are lacking information on generality of such fast multi-wavelength variability. We aim to undertake a multi- wavelength variability study of XRBs using multi-band high time resolution instrument such as HiPERFCAM and ULTRACAM.

Our aims are as follows:

1. undertake a study to quantify the optical/near-IR/X-ray/radio timing properties of NS and BH XRBs. Such data are crucial for disentangling the emission from various accretion components and allows us to probe the interaction between the accretion disc and corona and determine the role played by jets.

2.characterize the location of the optical/X-ray time delay in BH-XRBs, which gives the physical scale over which the plasma is accelerated in the inner jet, placing strong constraints on both internal shock and magneto-hydrodynamic models. 3.perform time delays studies of the optical pulsations observed in XRBs and binary millisecond pulsars to probe the nature of warped accretion disc and/or the motions of the secondary star and determine the nature of the channelled inner accretion flow onto the NSs in binary millisecond pulsars.