  We present a study aiming at detecting VMS in local star-forming region from
the imprint they leave on the integrated UV and optical light. We analyzed a
sample of 27 star-forming regions and galaxies in the local Universe. We
selected sources with a metallicity close to that of the LMC. We defined
empirical criteria to distinguish sources dominated by VMS and Wolf-Rayet stars
(WR), using template spectra of VMS- and WR-dominated regions. We subsequently
built population synthesis models with an updated treatment of VMS. We show
that the UV range alone is not sufficient to distinguish between VMS- and
WR-dominated sources. The region of the WR bumps in the optical breaks the
degeneracy. In particular, the morphology of the blue bump at 4640-4686 A is a
key diagnostic. Beyond the prototypical R136 region we identify two galaxies
showing clear signatures of VMS. In two other galaxies or regions the presence
of VMS can be suspected, as already discussed in the literature. The stellar
population is clearly dominated by WR stars in seven other sources. The most
recent BPASS population synthesis models can neither account for the strong
HeII 1640 emission, nor for the shape of the blue bump in VMS- and WR-dominated
sources. Our models that include VMS more realistically reproduce the
UV-optical spectra of VMS-dominated sources. We conclude that VMS are present
in some local star-forming regions, but that separating them from WR-dominated
populations requires optical spectroscopy with a high signal-to-noise ratio. A
high equivalent width of HeII 1640 is not a sufficient condition for
identifying VMS. Populations synthesis models need to take VMS into account by
incorporating not only evolutionary tracks, but also dedicated spectral
libraries. Finally, we stress that the treatment of WR stars needs to be
improved as well.
