We use Far Ultraviolet Spectrocopic Explorer ( FUSE ) observations to study interstellar absorption along the line of sight to the white dwarf WD 1634-573 ( d = 37.1 \pm 2.6 pc ) .
Combining our measurement of D I with a measurement of H I from Extreme Ultraviolet Explorer data , we find a D/H ratio toward WD 1634-573 of { D / H } = ( 1.6 \pm 0.5 ) \times 10 ^ { -5 } .
In contrast , multiplying our measurements of { D~ { } I / O~ { } I } = 0.035 \pm 0.006 and { D~ { } I / N~ { } I } = 0.27 \pm 0.05 with published mean Galactic ISM gas phase O/H and N/H ratios yields { D / H } _ { O } = ( 1.2 \pm 0.2 ) \times 10 ^ { -5 } and { D / H } _ { N } = ( 2.0 \pm 0.4 ) \times 10 ^ { -5 } , respectively .
Note that all uncertainties quoted above are 2 \sigma .
The inconsistency between { D / H } _ { O } and { D / H } _ { N } suggests that either the O I/H I and/or the N I/H I ratio toward WD 1634-573 must be different from the previously measured average ISM O/H and N/H values .
The computation of { D / H } _ { N } from D I/N I is more suspect , since the relative N and H ionization states could conceivably vary within the LISM , while the O and H ionization states will be more tightly coupled by charge exchange .