  We present the ultraviolet to submillimetre spectral energy distributions
(SEDs) of the HERschel Ultra Luminous Infrared Galaxy Survey (HERUS) sample of
42 local ultraluminous infrared galaxies (ULIRGs) and fit them with a Markov
chain Monte Carlo (MCMC) code using the CYprus models for Galaxies and their
NUclear Spectra (CYGNUS) radiative transfer models for starbursts, active
galactic nucleus (AGN) tori and host galaxy. The Spitzer IRS spectroscopy data
are included in the fitting. Our bayesian SED fitting method takes comparable
time to popular energy balance methods but it is more physically motivated and
versatile. All HERUS galaxies harbor high rates of star formation but we also
find bolometrically significant AGN in all of the galaxies of the sample. We
estimate the correction of the luminosities of the AGN in the ULIRGs due to the
anisotropic emission of the torus and find that it could be up to a factor of
$\sim10$ for nearly edge-on tori. We present a comparison of our results with
the smooth torus model of Fritz et al. and the two-phase models of Siebenmorgen
et al. and SKIRTOR. We find that the CYGNUS AGN torus models fit significantly
better the SEDs of our sample compared to all other models. We find no evidence
that strong AGN appear either at the beginning or end of a starburst episode or
that starbursts and AGN affect each other. IRAS 01003-2238 and Mrk 1014 show
evidence for dual AGN in their SED fits suggesting a minimum dual AGN fraction
in the sample of 5%.
