We present HST WFPC2 I band imaging for a sample of 9 Hyperluminous Infrared
Galaxies spanning a redshift range 0.45 < z < 1.34. Three of the sample have
morphologies showing evidence for interactions, six are QSOs. Host galaxies in
the QSOs are reliably detected out to z ~ 0.8. The detected QSO host galaxies
have an elliptical morphology with scalelengths spanning 6.5 < r_{e}(Kpc) < 88
and absolute k corrected magnitudes spanning -24.5 < M_{I} < -25.2. There is no
clear correlation between the IR power source and the optical morphology. None
of the sources in the sample, including F15307+3252, show any evidence for
gravitational lensing. We infer that the IR luminosities are thus real. Based
on these results, and previous studies of HLIRGs, we conclude that this class
of object is broadly consistent with being a simple extrapolation of the ULIRG
population to higher luminosities; ULIRGs being mainly violently interacting
systems powered by starbursts and/or AGN. Only a small number of sources whose
infrared luminosities exceed 10^{13}Lsun are intrinsically less luminous
objects which have been boosted by gravitational lensing.Comment: 16 Pages. Accepted for publication in MNRA
