  We here report the detection of the nebular HeII4686 line in 32 HII regions
in the metal-poor collisional ring galaxy Cartwheel using the Multi-Unit
Spectroscopic Explorer (MUSE) dataset. The measured I(HeII4686)/I(Hbeta) ratio
varies from 0.004 to 0.07, with a mean value of 0.010+/-0.003. Ten of these 32
HII regions are coincident with the location of an Ultra Luminous X-ray (ULX)
source. We used the flux ratios of important diagnostic lines and results of
photoionization by Simple Stellar Populations (SSPs) to investigate the likely
physical mechanisms responsible for the ionization of He+. We find that the
majority of the regions (27) are consistent with photoionization by star
clusters in their Wolf-Rayet (WR) phase with initial ionization parameter
-3.5<logU<-2.0. Blue Bump (BB), the characteristic feature of the WR stars,
however, is not detected in any of the spectra. We demonstrate that this
non-detection is due to the relatively low equivalent width (EW) of the BB in
metal-poor SSPs, in spite of containing sufficient number of WR stars to
reproduce the observed I(HeII4686)/I(Hbeta) ratio of 1.5% at the Cartwheel
metallicity of Z=0.004. The HII regions in the WR phase that are coincident
with a ULX source do not show line ratios characteristic of ionization by X-ray
sources. However, the ULX sources may have a role to play in the ionization of
He+ in two (#99, 144) of the five regions that are not in the WR phase.
Ionization by radiative shocks along with the presence of channels for the
selective leakage of ionizing photons are the likely scenarios in #17 and #148,
the two regions with the highest observed I(HeII4686)/I(Hbeta) ratio.
