  We present the results of a multiwavelength survey of HI-excess galaxies, an
intriguing population with large HI reservoirs associated with little current
star formation. These galaxies have stellar masses $M_{\star} >10^{10}$
M$_{\odot}$, and were identified as outliers in the gas fraction vs. NUV$-r$
color and stellar mass surface density scaling relations based on the GALEX
Arecibo SDSS Survey (GASS). We obtained HI interferometry with the GMRT, Keck
optical long-slit spectroscopy and deep optical imaging (where available) for
four galaxies. Our analysis reveals multiple possible reasons for the HI excess
in these systems. One galaxy, AGC 10111, shows an HI disk that is
counter-rotating with respect to the stellar bulge, a clear indication of
external origin of the gas. Another galaxy appears to host a Malin 1-type disk,
where a large specific angular momentum has to be invoked to explain the
extreme $M_{\rm HI}$/$M_{\star}$ ratio of 166$\%$. The other two galaxies have
early-type morphology with very high gas fractions. The lack of merger
signatures (unsettled gas, stellar shells and streams) in these systems
suggests that these gas-rich disks have been built several Gyr-s ago, but it
remains unclear how the gas reservoirs were assembled. Numerical simulations of
large cosmological volumes are needed to gain insight into the formation of
these rare and interesting systems.
