  Cosmogenic neutrinos are produced when ultra-high-energy cosmic rays (UHECRs)
interact with cosmological photon fields. Limits on the diffuse flux of these
neutrinos can be used to constrain the fraction of protons arriving at Earth
with energies $E_{p} \gtrsim 30$ EeV, thereby providing constraints on the
composition of UHECRs without fully relying on hadronic interaction models. We
show to which extent current neutrino telescopes already constrain this
fraction of protons and discuss the prospects for next-generation detectors to
further constrain it. Additionally, we discuss the implications of these limits
for several popular candidates for UHECR source classes.
