  The fundamental band for the OC-C2H2 dimer and two combination bands
involving the intermolecular bending modes nu9 and nu8 in the carbon monoxide
CO stretch region are re-examined. Spectra are obtained using a pulsed
supersonic slit jet expansion probed with a mode-hop free tunable infrared
quantum cascade laser. Analogous bands for OC-C2D2 and the fundamental for
OC-DCCH as an impurity are also observed and analysed. A much weaker band in
the same spectral region is assigned to a new mixed trimer, CO-(C2H2)2. The
trimer band is composed uniquely of a-type transitions, establishing that the
CO monomer is nearly aligned with the a-inertial axis. The observed rotational
constants agree well with ab initio calculations and a small inertial defect
value indicates that the trimer is planar. The structure is a compromise
between the T-shaped structure of free acetylene dimer and the linear geometry
of free OC-C2H2. A similar band for the fully deuterated isotopologue
CO-(C2D2)2 confirms our assignment.
