  Young, massive stars dominate the rest-frame ultraviolet spectra of
star-forming galaxies. At high redshifts (z>2), these rest-UV features are
shifted into the observed-frame optical and a combination of gravitational
lensing, deep spectroscopy and spectral stacking analysis allows the stellar
population characteristics of these sources to be investigated. We use our
stellar population synthesis code BPASS to fit two strong rest-UV spectral
features in published Lyman-break galaxy spectra, taking into account the
effects of binary evolution on the stellar spectrum. In particular, we consider
the effects of quasi-homogeneous evolution (arising from the rotational mixing
of rapidly-rotating stars), metallicity and the relative abundance of carbon
and oxygen on the observed strengths of HeII (1640 Angstroms) and CIV
(1548,1551 Angstroms) spectral lines. We find that Lyman-break galaxy spectra
at z=2-3 are best fit with moderately sub-solar metallicities, and with a
depleted carbon-to-oxygen ratio. We also find that the spectra of the lowest
metallicity sources are best fit with model spectra in which the HeII emission
line is boosted by the inclusion of the effect of massive stars being spun-up
during binary mass-transfer so these rapidly-rotating stars experiencing
quasi-homogeneous evolution.
