  The optical spectra of objects classified as QSOs in the SDSS DR6 are
analyzed with the aim of determining the value of the fine structure constant
in the past and then check for possible changes in the constant over
cosmological timescales. The analysis is done by measuring the position of the
fine structure lines of the [OIII] doublet (4959 and 5008) in QSO nebular
emission. From the sample of QSOs at redshifts z < 0.8 a subsample was selected
on the basis of the amplitude and width of the [OIII] lines. Two different
method were used to determine the position of the lines of the [OIII] doublet,
both giving similar results. Using a clean sample containing 1568 of such
spectra, a value of Delta alpha /alpha=(+2.4 +-2.5) x 10^{-5} (in the range of
redshifts 0-0.8) was determined. The use of a larger number of spectra allows a
factor ~5 improvement on previous constraints based on the same method. On the
whole, we find no evidence of changes in alpha on such cosmological timescales.
The mean variation compatible with our results is 1/ <t> Delta
alpha/alpha=(+0.7 +- 0.7) x 10^{-14} yr^{-1}. The analysis was extended to the
[NeIII] and [SII] doublets, although their usefulness is limited due to the
fact that all these doublets in QSOs tend to be fainter than [OIII], and that
some of them are affected by systematics.
