  We present a method to determine the proton-to-helium ratio in cosmic rays at
ultra-high energies. It makes use of the exponential slope, $\Lambda$, of the
tail of the $X_{\rm max}$ distribution measured by an air shower experiment.
The method is quite robust with respect to uncertainties from modeling hadronic
interactions and to systematic errors on $X_{\rm max}$ and energy, and to the
possible presence of primary nuclei heavier than helium. Obtaining the
proton-to-helium ratio with air shower experiments would be a remarkable
achievement.
  To quantify the applicability of a particular mass-sensitive variable for
mass composition analysis despite hadronic uncertainties we introduce as a
metric the `analysis indicator' and find an improved performance of the
$\Lambda$ method compared to other variables currently used in the literature.
The fraction of events in the tail of the $X_{\rm max}$ distribution can
provide additional information on the presence of nuclei heavier than helium in
the primary beam.
