  (Abridged) Astronomers recently started discovering exoplanets around binary
systems. Therefore, understanding the formation and evolution of circumbinary
disks is crucial for a complete scenario of planet formation. The aim of this
paper is to present the detection of a circumbinary disk around Oph-IRS67 and
analyse its structure. We present high-angular-resolution (0.4", 60 AU)
observations of C17O, H13CO+ , C34S, SO2, C2H and c-C3H2 molecular transitions
with ALMA at 0.8 mm. The spectrally and spatially resolved maps reveal the
kinematics of the circumbinary disk as well as its chemistry. Molecular
abundances are estimated using RADEX. The continuum emission reveals the
presence of a circumbinary disk around the two sources. This disk has a
diameter of ~620 AU and is well traced by C17O and H13CO+ emission. C2H and
c-C3H2 trace a higher-density region which is spatially offset from the sources
(~430 AU). Finally, SO2 shows compact emission around one of the sources,
Oph-IRS67 B. The molecular transitions which trace the circumbinary disk are
consistent with a Keplerian profile on disk scales (< 200 AU) and an infalling
profile for envelope scales (> 200 AU). The Keplerian fit leads to a mass of
2.2 Msun. Inferred CO abundances w.r.t. H2 are comparable to the canonical ISM
value of 2.7e-4. This study proves the first detection of the circumbinary disk
associated with Oph-IRS67. The disk is chemically differentiated from the
nearby high-density region. The lack of methanol emission suggests the extended
disk dominates the mass budget in the inner- most regions of the protostellar
envelope, generating a flat density profile where less material is exposed to
high temperatures. Thus, complex organic molecules would be associated with
lower column densities. Finally, Oph-IRS67 is a promising candidate for the
detection of both circumstellar disks with higher-angular-resolution
observations.
