There are three low-mass X-ray binaries (4U 0614+091, 2S 0918-549, and 4U 1
543-624) for which broadline emission near 0.7 keV was previously reported.
A recent high-resolution observation of 4U 0614+091 with the Chandra Low-E
nergy Transmission Grating Spectrometer (LETGS) found evidence of an unusua
lly high Ne/O abundance ratio along the line of sight but failed to detect
the previously reported 0.7 keV feature. We have made a search of archival
ASCA spectra and identified a fourth source with the 0.7 keV feature, the 2
0 minute ultracompact binary 4U 1850-087. In all four of these sources, the
0.7 keV residual is eliminated from the ASCA spectra by allowing excess ph
otoelectric absorption due to a nonsolar relative abundance of neon, just a
s in the LETGS spectrum of 4U 0614+091. The optical properties of these sys
tems suggest that all four are ultracompact (P-orb < minutes) binaries. We
propose that there is excess neon local to each of these sources, as also f
ound in the ultracompact binary pulsar 4U 1626-67. We suggest that the mass
donor in these systems is a low-mass, neon-rich degenerate dwarf and that
the binaries are all ultracompact.