  We present an emission-line diagnostic analysis of integral-field
spectroscopic observations that cover the central kiloparsec of NGC 7742. This
Sa galaxy hosts a spectacular nuclear starburst ring and nuclear regions
characterized by low-ionization emission. The gas in the ring rotates in the
opposite sense to the stars in the galaxy, suggesting a recent merging or
acquisition event. The combination of integral-field measurements for the
H-alpha + [NII] emission lines from DensePak and the H-beta and [OIII] emission
from Sauron allow the construction of diagnostic diagrams that highlight the
transition from star formation in the nuclear ring to excitation by
high-velocity shocks or by a central AGN towards the center. DensePak
measurements for the [SII] line ratio reveal very low gas densities in the
nuclear ring, Ne < 100 per cm^3, characteristic of massive HII regions.
Comparison with MAPPINGS III models for starbursts with low gas densities show
that the ring is of roughly solar metallicity. This suggests that the gas in
the nuclear ring originated in a stellar system capable of substantially
enriching the gas metallicity through sustained star formation. We suggest that
NGC 7742 cannibalised a smaller galaxy rich in metal-poor gas, and that star
formation episodes in the ring have since increased the metallicity to its
present value.
