  We present GMRT HI observations and deep CFHT MegaCam optical images of the
gas-rich interacting galaxy group NGC 871/NGC 876/NGC 877 (hereafter NGC
871/6/7). Our high-resolution data sets provide a census of the HI and stellar
properties of the detected gas-rich group members. In addition to a handful of
spiral, irregular and dwarf galaxies, this group harbours an intriguing HI
feature, AGC 749170, that has a gas mass of ~10^9.3 M_sol, a dynamical-to-gas
mass ratio of ~1 (assuming the cloud is rotating and in dynamical equilibrium)
and no optical counterpart in previous imaging. Our observations have revealed
a faint feature in the CFHT g'- and r'-bands; if it is physically associated
with AGC 749170, the latter has M/L_g > 1000 M_sol/L_sol as well as a higher
metallicity (estimated using photometric colours) and a significantly younger
stellar population than the other low-mass gas-rich group members. These
properties, as well as its spectral and spatial location with respect to its
suspected parent galaxies, strongly indicate a tidal origin for AGC 749170.
Overall, the HI properties of AGC 749170 resemble those of other optically
dark/dim clouds that have been found in groups. These clouds could represent a
class of relatively long-lived HI-rich tidal remnants that survive in
intermediate-density environments.
