  Disentangling techniques are often needed to obtain the spectra of the
individual components of binary or multiple systems. A thorough analysis of the
shift-and-add algorithm of Marchenko, Moffat, & Eenens (1998) reveals that in
many cases the line fluxes are poorly reproduced and spurious wings appear. The
causes of these discrepancies are discussed and a new disentangling package,
QER20, is presented which significantly reduces these errors and vastly
increases the performance. When applied to the massive binary 9 Sgr, our new
code yields line fluxes which are notably different from those previously
published and lead us to revise the spectral classification to slightly earlier
subtypes: O3V((f +)) for the primary and O5V((f)) for the secondary. We show
that with the MME98 algorithm the classification of massive stars in binaries
can be off by several subtypes whilst there are no such errors when the QER20
package is used.
