  The study of young stellar populations has revealed that most stars are in
binary or higher order multiple systems. In this study the influence on the
stellar initial mass function (IMF) of large quantities of unresolved multiple
massive stars is investigated by taking into account stellar evolution and
photometrically determined system masses. The models where initial masses are
derived from the luminosity and colour of unresolved multiple systems show that
even under extreme circumstances (100% binaries or higher order multiples) the
difference between the power-law index of the mass function of all stars and
the observed mass function is small (~0.1). Thus, if the observed IMF has the
Salpeter index alpha = 2.35 then the true stellar IMF has an index not flatter
than alpha = 2.25. Additionally, unresolved multiple systems may hide between
15 and 60% of the underlying true mass of a star cluster. While already a known
result, it is important to point out that the presence of a large number of
unresolved binaries amongst pre-main-sequence (PMS) stars induces a significant
spread in the measured ages of these stars even if there is none. Also,
lower-mass stars in a single-age binary-rich cluster appear older than the
massive stars by about 0.6 Myr.
