  The origin of nebular HeII emission, which is frequently observed in
low-metallicity (O/H) star-forming galaxies, remains largely an unsolved
question. Using the observed anticorrelation of the integrated X-ray luminosity
per unit of star formation rate ($L_X/{\rm SFR}$) of an X-ray binary population
with metallicity and other empirical data from the well-studied galaxy I Zw 18,
we show that the observed HeII 4686 intensity and its trend with metallicity is
naturally reproduced if the bulk of He$^+$ ionizing photons are emitted by the
X-ray sources. We also show that a combination of X-ray binary population
models with normal single and/or binary stellar models reproduces the observed
$I(4686)/I(H\beta)$ intensities and its dependency on metallicity and age. We
conclude that both empirical data and theoretical models suggest that high-mass
X-ray binaries are the main source of nebular HeII emission in low-metallicity
star-forming galaxies.
