  We examine the merger stage of IRAS 03158+4227, one of the most luminous
ULIRGs from the IRAS 2 Jy sample. Deep optical images are presented along with
high-resolution NIR images and optical low-resolution spectra. IRAS 03158+4227
is confirmed as a component of an equal-luminosity binary galaxy with a nuclei
separation of 47 / (h/75) kpc. A long lopsided tail emanating from the
companion, which harbours an active nucleus, is the most prominent peculiar
feature of the binary. The results of numerical simulations admit the
interpretation of this structure as a product of the tidal interaction between
the two components. If the infrared-activity of IRAS 03158+4227 is also
dynamically triggered by this process, this would implicate that the ULIRG
phenomenon is not restricted to the final stage of a binary merger.
Alternatively, the system may be a multiple merger where the partner(s)
has/have already coalesced.
