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 \ion H1 column density to model the absorption .
The HST spectrum shows an asymmetric Lyman- \alpha ( Ly \alpha ) feature which is produced by either \ion H1 at a second velocity , or a high abundance of D. Three models with a single simple H+D component give 8 \times 10 ^ { -5 } < D / H < 57 \times 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 \times 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 \ion H1 column densities have more than one component .