ar X iv : a st ro - p h / 98 10 21 7 v 1 1 4 O ct 1 99 8 The Deuterium Abundance at z = 0 .

We present constraints on the deuterium to hydrogen ratio (D/H) in the metal-poor gas cloud at redshift z = 0.701 towards QSO 1718+4807. We use new Keck spectra in addition to Hubble Space Telescope (HST) and International Ultraviolet Explorer (IUE) spectra. We use an improved redshift and a lower H I column density to model the absorption. The HST spectrum shows an asymmetric Lyman-α (Lyα) feature which is produced by either H I at a second velocity, or a high abundance of D. Three models with a single simple H+D component give 8× 10−5 < D/H < 57× 10−5 (95%), a much larger range than reported by Webb et al. (1997a,b). A more sophisticated velocity distribution, or a second component is necessary for lower D/H. With two components, which could be a part of one absorbing structure, or separate clouds in a galaxy halo, we find D/H < 50 × 10−5. We do not know if this second component is present, but it is reasonable because 40 – 100% of absorption systems with similar redshifts and H I column densities have more than one component. Subject headings: quasars: absorption lines — quasars: individual (Q1718+4807) Based on observations obtained with the NASA/ESA Hubble Space Telescope obtained by the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. Visiting Astronomer, W. M. Keck Telescope, California Association for Research in Astronomy Present address: University of Chicago, Dept. of Astronomy & Astrophysics, 5640 S. Ellis Ave., Chicago, IL 60637