  Hyperluminous infrared galaxies (HLIRG) are the most luminous persistent
objects in the Universe. They exhibit extremely high star formation rates, and
most of them seem to harbour an AGN. They are unique laboratories to
investigate the most extreme star formation, and its connection to
super-massive black hole growth. The AGN and SB relative contributions to the
total output in these objects is still debated. Our aim is to disentangle the
AGN and SB emission of a sample of thirteen HLIRG. We have studied the MIR low
resolution spectra of a sample of thirteen HLIRG obtained with the IRS on board
Spitzer. The 5-8 {\mu}m range is an optimal window to detect AGN activity even
in a heavily obscured environment. We performed a SB/AGN decomposition of the
continuum using templates, successfully applied for ULIRG in previous works.
The MIR spectra of all sources is largely dominated by AGN emission. Converting
the 6 {\mu}m luminosity into IR luminosity, we found that ~80% of the sample
shows an IR output dominated by the AGN emission. However, the SB activity is
significant in all sources (mean SB contribution ~30%), showing star formation
rates ~300-3000 solar masses per year. Using X-ray and MIR data we estimated
the dust covering factor (CF) of these HLIRG, finding that a significant
fraction presents a CF consistent with unity. Along with the high X-ray
absorption shown by these sources, this suggests that large amounts of dust and
gas enshroud the nucleus of these HLIRG, as also observed in ULIRG. Our results
are in agreement with previous studies of the IR SED of HLIRG using radiative
transfer models, and we find strong evidence that all HLIRG harbour an AGN.
This work provides further support to the idea that AGN and SB are both crucial
to understand the properties of HLIRG. Our study of the CF supports the
hypothesis that HLIRG can be divided in two different populations.
