  We present in this contribution our set of multiwavelength synthesis models
including the evolution of single and binary stars. The main results we have
obtained can be summarized as follows:
  (a) massive close-binary systems will start to experience mass transfer
episodes after the first 4 Myr of the starburst evolution;
  (b) as a result of these mass transfer processes, stars of relatively low
initial mass can loose completely their envelope and become a WR. In this way,
the formation of WR stars is extended over longer than 15 Myr, and does not
stop at 6 Myr as predicted by models including only single stars;
  (c) WR stars can thus be coeval with red supergiants, which peak at around 10
Myr for solar metallicities;
  (d) the accretion of mass will originate relatively massive stars at ages for
which they should have already disappeared; these stars, together with the WR
stars formed in rather evolved clusters, increase the production of ionizing
photons, so that the H$\beta$ equivalent width will not drop as rapidly as
predicted by models considering only individual stars; and
  (e) the mass transfer to compact companions will produce an additional source
of high-energy radiation in the form of high-mass X-ray binaries, not predicted
either by standard synthesis models.
