  In the complete sample of ultraluminous infrared galaxies (ULIRGs) compiled
by Kim (1995) about 5% consists of widely separated galaxies which are
presumably in the early phase of interaction. This fact is contrary to the
conventional view that ULIRGs are in the final stages of the merger of two
gas-rich disk galaxies. We have undertaken high resolution CO(J=1-0)
observations for the ultraluminous infrared galaxies that have nuclear
separations larger than 20 kpc. We have detected the CO emission in 5 out of 6
systems, but only in one component of the ULIRG pairs. 4 of them have LINER
spectral type and 1 galaxy has Seyfert II spectral type. In K'-band images
these components are also brighter than the other components which have either
HII-region spectra or no detectable emission lines. Using the standard
conversion factor, the molecular gas content is estimated to be a few times 10
$^{10}$ M$_\odot$, similar to that of the other ultraluminous galaxies. The
result indicates that the galaxy containing the molecular gas is also the
source of most, if not all, of the huge far-infrared luminosity of the system.
The optical and K'-band imaging observations and optical spectra suggest
multiple merger scenario for 1 system. If the remaining systems are in an early
stage of a binary tidal interaction, the commonly accepted interpretation of
the ULIRG phenomenon as the final merger stage of two disk galaxies may need to
be re-examined.
