  We report on first results of computing synthetic spectra from H/He-poor
accretion disks in ultracompact LMXBs. We aim at the determination of the
chemical composition of the very low-mass donor star, which is the core of a
former C/O white dwarf. The abundance analysis allows to draw conclusions on
gravitational settling in WDs which is an important process affecting cooling
times and pulsational g-mode periods.
