  The continuum shape of 26 broad absorption line (BAL) QSOs is shown to be
virtually identical to that of non-BAL QSOs when the spectra of the former are
dereddened. A procedure for dereddening of BAL QSO spectra is introduced, based
on measurement of the flux in three bands and determination of the colour
excess in comparison with an unreddened spectrum. This method allows us to
derive the type and amount of extinction. We show that reddening in
high-ionization BAL QSOs is comparatively small and the range of extinction
laws varies from Milky Way, LMC to SMC type. On the other hand, low-ionization
BAL QSOs seem to experience reddening which can be attributed to SMC type
extinction alone. Comparison of the colour indices of composite spectra from
Francis etal. (1991) and Zheng etal. (1997) with those derived from individual
BAL QSO spectra shows that the Francis etal. (1991) composite might be a more
suitable representative of an unreddened spectrum. In general, we conclude that
the spectral energy distribution of normal QSOs can be used as an incident
continuum shape for BAL QSOs.
