The most metal-poor DLA known to date , at z _ { abs } = 2.61843 in the spectrum of the QSO Q0913+072 , with an oxygen abundance only \sim 1 / 250 of the solar value , shows six well resolved D i Lyman series transitions in high quality echelle spectra recently obtained with the ESO VLT .
We deduce a value of the deuterium abundance \log { ( D / H ) } = -4.56 \pm 0.04 which is in good agreement with four out of the six most reliable previous determinations of this ratio in QSO absorbers .
We find plausible reasons why in the other two cases the 1 \sigma errors may have been underestimated by about a factor of two .
The addition of this latest data point does not change significantly the mean value of the primordial abundance of deuterium , suggesting that we are now converging to a reliable measure of this quantity .
We conclude that \langle \log { ( D / H ) _ { p } } \rangle = -4.55 \pm 0.03 and \Omega _ { b, 0 } h ^ { 2 } { ( BBN ) } = 0.0213 \pm 0.0010 ( 68 % confidence limits ) .
Including the latter as a prior in the analysis of the five year data of WMAP leads to a revised best-fitting value of the power-law index of primordial fluctuations n _ { s } = 0.956 \pm 0.013 ( 1 \sigma ) and n _ { s } < 0.990 with 99 % confidence .
Considering together the constraints provided by WMAP 5 , ( D/H ) _ { p } , baryon oscillations in the galaxy distribution , and distances to Type Ia supernovae , we arrive at the current best estimates \Omega _ { b, 0 } h ^ { 2 } = 0.0224 \pm 0.0005 and n _ { s } = 0.959 \pm 0.013 .