  Measurements of the atmospheric carbon (C) and oxygen (O) relative to
hydrogen (H) in hot Jupiters (relative to their host stars) provide insight
into their formation location and subsequent orbital migration. Hot Jupiters
that form beyond the major volatile (H2O/CO/CO2) ice lines and subsequently
migrate post disk-dissipation are predicted have atmospheric carbon-to-oxygen
ratios (C/O) near 1 and subsolar metallicities, whereas planets that migrate
through the disk before dissipation are predicted to be heavily polluted by
infalling O-rich icy planetesimals, resulting in C/O < 0.5 and super-solar
metallicities. Previous observations of hot Jupiters have been able to provide
bounded constraints on either H2O or CO, but not both for the same planet,
leaving uncertain the true elemental C and O inventory and subsequent C/O and
metallicity determinations. Here we report spectroscopic observations of a
typical transiting hot Jupiter, WASP-77Ab. From these, we determine the
atmospheric gas volume mixing ratio constraints on both H2O and CO (9.5$\times
10^{-5}$ - 1.5$\times 10^{-4}$ and 1.2$\times 10^{-4}$ - 2.6$\times 10^{-4}$,
respectively). From these bounded constraints, we are able to derive the
atmospheric C/H (0.35$^{+0.17}_{-0.10}$ $\times$ Solar) and O/H (0.32
$^{+0.12}_{-0.08}$ $\times$ Solar) abundances and the corresponding atmospheric
carbon-to-oxygen ratio (C/O=0.59$\pm$0.08; the solar value is 0.55). The
sub-solar (C+O)/H (0.33$^{+0.13}_{-0.09}$ $\times$ Solar) is suggestive of a
metal-depleted atmosphere relative to what is expected for Jovian-like planets
while the near solar value of C/O rules out the disk-free migration/C-rich
atmosphere scenario.
