  The most metal-poor DLA known to date, at z = 2.61843 in the spectrum of the
QSO Q0913+072, with an oxygen abundance only about 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+/-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 <log (D/H)_p> = -4.55+/-0.03 and
Omega_b h^2 (BBN) = 0.0213+/-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+/-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 h^2 = 0.0224+/-0.0005 and n_s = 0.959+/-0.013.
