  We present first results from our Very Large Telescope large program to study
the dynamical evolution of Ultraluminous Infrared Galaxies (ULIRGs), which are
the products of mergers of gas-rich galaxies. The full data set consists of
high resolution, long-slit, H- and K-band spectra of 38 ULIRGs and 12 QSOs
(between 0.042<z<0.268). In this paper, we present the sources that have not
fully coalesced, and therefore have two distinct nuclei. This sub-sample
consists of 21 ULIRGs, the nuclear separation of which varies between 1.6 and
23.3 kpc. From the CO bandheads that appear in our spectra, we extract the
stellar velocity dispersion, sigma, and the rotational velocity, V_rot. The
stellar dispersion equals 142 km/s on average, while V_rot is often of the same
order. We combine our spectroscopic results with high-resolution infrared (IR)
imaging data to study the conditions for ULIRG activity in interacting pairs.
We find that the majority of ULIRGs are triggered by almost equal-mass major
mergers of 1.5:1 average ratio. Less frequently, 3:1 encounters are also
observed in our sample. However, less violent mergers of mass ratio >3:1
typically do not force enough gas into the center to generate ULIRG
luminosities.
