The NIRVANDELS Survey: a robust detection of $\alpha$-enhancement in
  star-forming galaxies at $z\simeq3.4$
  We present results from the NIRVANDELS survey investigating the gas-phase
metallicity ($\mathrm{Z}_{\mathrm{gas}}$, tracing O/H) and stellar metallicity
($Z_{\star}$, tracing Fe/H) of 33 star-forming galaxies at redshifts $2.95 < z
< 3.80$. Based on a combined analysis of deep optical and near-IR spectra,
tracing the rest-frame far ultraviolet and rest-frame optical respectively, we
present the first simultaneous determination of the stellar and gas-phase
mass-metallicity relationships (MZRs) at $z\simeq3.4$. In both cases, we find
that metallicity increases with increasing stellar mass ($M_{\star}$), and that
the power-law slope at $M_{\star} \lesssim 10^{10} \mathrm{M}_{\odot}$ of both
MZRs scales as $Z \propto M_{\star}^{0.3}$. Comparing the stellar and gas-phase
MZRs, we present direct evidence for super-solar O/Fe ratios (i.e.,
$\alpha$-enhancement) at $z>3$, finding $\mathrm{(O/Fe)}\simeq (2.54 \pm 0.38)
\times \mathrm{(O/Fe)}_{\odot}$, with no clear dependence on $M_{\star}$.
