  We present new medium-resolution high-S/N optical spectra of the ultracompact
low-mass X-ray binaries 4U0614+091 and 4U1626-67, taken with the ESO Very Large
Telescope. They are pure emission line spectra and the lines are identified as
due to C II-IV and O II-III Line identification is corroborated by first
results from modeling the disk spectra with detailed non-LTE radiation transfer
calculations. Hydrogen and helium lines are lacking in the observed spectra.
Our models confirm the deficiency of H and He in the disks. The lack of neon
lines suggests an Ne abundance of less than about 10 percent (by mass),
however, this result is uncertain due to possible shortcomings in the model
atom. These findings suggest that the donor stars are eroded cores of C/O white
dwarfs with no excessive neon overabundance. This would contradict earlier
claims of Ne enrichment concluded from X-ray observations of circumbinary
material, which was explained by crystallization and fractionation of the white
dwarf core.
