  The average optical-UV continuum shape of QSOs has been investigated using
spectra of 62 QSOs having good relative photometric calibrations. The QSO
spectra were extracted from two complete color selected samples in the
magnitude intervals B ~ 16-20. The analysis was performed fitting power-law
continua (f proportional to nu^(alpha)) in well defined rest-frame wavelength
intervals after removing regions of the spectrum affected by strong emission
lines or weak emission bumps. The average slope in the rest-frame optical-UV
region 1200 - 5500 A shows a rapid change around the 3000 A emission bump with
alpha=0.15 longward of it and alpha=-0.65 at shorter wavelengths. Although
these average slopes have been obtained using spectra of QSOs with different
luminosities and redshifts, there are no significant correlations of the
average spectral index with these quantities. For a few QSOs in the sample we
were able to measure the same softening of the spectral shape within the
individual spectrum. These results have significant consequences on the
estimate of the cosmological evolution of the optically selected QSOs as they
affect, for instance, the k-corrections. New k-corrections in the B, V, R and
Gr bands were computed. The derived average spectral shape in the optical-UV
band puts interesting constraints on the expected emission mechanisms.
