The Deuterium Abundance at [FORMULA][F]z=0.701[/F][/FORMULA][CLC][ITAL]z[/ITAL][/CLC] = 0.701 toward QSO 1718+4807

We present constraints on the deuterium to hydrogen ratio (D/H) in themetal-poor gas cloud at redshift $z=0.701$ towards QSO 1718+4807. We use newKeck spectra in addition to Hubble Space Telescope (HST) and InternationalUltraviolet Explorer (IUE) spectra. We use an improved redshift and a lower \HIcolumn density to model the absorption. The HST spectrum shows an asymmetricLyman-$\alpha$ (\lya) feature which is produced by either \HI at a secondvelocity, or a high abundance of D. Three models with a single simple H+Dcomponent give $8 \times 10^{-5} < D/H < 57 \times 10^{-5}$ (95%), a muchlarger range than reported by Webb et al (1997a,b). A more sophisticatedvelocity distribution, or a second component is necessary for lower D/H. Withtwo components, which could be a part of one absorbing structure, or separateclouds in a galaxy halo, we find $D/H < 50 \times 10^{-5}$. We do not know ifthis second component is present, but it is reasonable because 40 -- 100% ofabsorption systems with similar redshifts and \HI column densities have morethan one component.Comment: 13 pages, 4 figures, to appear in the Astronomical Journal (Jan 1999