The vast majority of the known galactic population of stellar-mass black holes and a significant fraction of the neutron stars are found in X-ray binary systems. Among them, the population of ultra-compact X-ray binaries (UCXBs) is of particular interest, as they harbour degenerate donor stars that feed the black-hole/neutron star with hydrogen poor material. UCXBs are unique laboratories to study accretion processes in hydrogen deficient environments as well as some of the fundamental stages of binary evolution. In addition, since they are composed by two ultra-dense objects in a tight binary orbit, UCXBs play a crucial role in the new era of gravitational wave astronomy. As a matter of fact, they are expected to be primary sources for the detection of persistent gravitational waves. Several studies have shown that the observational properties of UCXBs place most of them within the newly discovered class of sub-luminous X-ray binaries, an intriguing group of objects in which the principal investigator (PI) is a recognised international expert.

This scientific project will combine the research team's expertise and international collaborations,  and the leadership of the team in optical and near-infrared studies of X-ray binaries. This project will aim at

(i) performing a global characterization of the general properties of UCXBs

(ii) significantly growing the known population of UCXBs by using novel observational techniques

(iii) establishing the contribution of UCBXs to the X-ray binary feedback into the interstellar medium

The superb access of IAC to world-class multi-wavelength facilities ensures that the project will make use of the best data, some of them already in hand, which together with the PI and research team expertise in the field should result in major advances in this novel topic of astrophysical research.