  Cosmogenic neutrinos are created when ultra-high-energy cosmic rays (UHECRs)
interact with extragalactic photon backgrounds. In general, the expected flux
of these cosmogenic neutrinos depends on multiple parameters, describing the
sources and propagation of UHECRs. In our recent paper (van Vliet et al. 2019),
we show that a `sweet spot` occurs at a neutrino energy of $E_{\nu} \sim 1$
EeV. At that energy the flux mainly depends on two parameters, the source
evolution and the fraction of protons in UHECRs at Earth for $E_p \gtrsim 30$
EeV. Therefore, with current upper limits on the cosmogenic neutrino flux at
$E_{\nu} \sim 1$ EeV and assuming a certain source class, a constraint on the
composition of UHECRs can be obtained. This constraint is independent of
hadronic interaction models and indicates that the combination of a large
proton fraction and a strong source evolution is disfavored. Upcoming neutrino
experiments will be able to constrain the fraction of protons in UHECRs even
further, and for any realistic model for the evolution of UHECR sources.
