The nature of the compact accretor in exotic X-ray binaries

Veryfaint Xray transients undergo accretion outbursts with a peak Xrayluminosity well below that of other black hole and neutron star lowmass Xraybinaries. These intriguing objects allow us to probe new parameter space forbinary evolution models, accretion flows, and neutron star physics. We proposeto study 2 veryfaint Xray transients in quiescence with XMMNewton (100 ks intotal) and HST (3 orbits in total). The aim of these observations are to i)elucidate the nature of their compact primary, ii) test if the Xray spectralevolution during outburst serves as a diagnostic to separate neutron stars fromblack holes and iii) provide the first detailed characterisation of thequiescent emission of veryfaint Xray transients. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]