We present high-resolution spectroscopy of the neutron star/low-mass X-ray binaries ( LMXBs ) 4U 1850 - 087 and 4U 0513 - 40 as part of our continuing study of known and candidate ultracompact binaries .
The LMXB 4U 1850 - 087 is one of four systems in which we had previously inferred an unusual Ne/O ratio in the absorption along the line of sight , most likely from material local to the binaries .
However , our recent Chandra X-ray Observatory LETGS spectrum of 4U 1850 - 087 finds a Ne/O ratio by number of 0.22 \pm 0.05 , smaller than previously measured and consistent with the expected interstellar value .
We propose that variations in the Ne/O ratio due to source variability , as previously observed in these sources , can explain the difference between the low- and high-resolution spectral results for 4U 1850 - 087 .
Our XMM-Newton RGS observation of 4U 0513 - 40 also shows no unusual abundance ratios in the absorption along the line of sight .
We also present spectral results from a third candidate ultracompact binary , 4U 1822 - 000 , whose spectrum is well fit by an absorbed power-law + blackbody model with absorption consistent with the expected interstellar value .
Finally , we present the non-detection of a fourth candidate ultracompact binary , 4U 1905 + 000 , with an upper limit on the source luminosity of < 1 \times 10 ^ { 32 } erg s ^ { -1 } .
Using archival data , we show that the source has entered an extended quiescent state .