Context : 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 Active Galactic Nucleus ( AGN ) .
They are unique laboratories to investigate the most extreme star formation , and its connection to super-massive black hole growth .

Aims : The AGN and starburst ( 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 .

Methods : We have studied the MIR low resolution spectra of a sample of thirteen HLIRG obtained with the Infrared Spectrograph on board Spitzer .
The 5 - 8 \upmu 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 , which has been successfully applied for ULIRG in previous works .

Results : The MIR spectra of all sources is largely dominated by AGN emission .
Converting the 6 \upmu m luminosity into IR luminosity , we found that \sim 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 \sim 30 \% ) , showing star formation rates \sim 300 - 3000 ~ { } M _ { \mathrm { \sun } } \mathrm { yr } ^ { -1 } .
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 .

Conclusions : 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 .
Moreover , 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 .