  We present the results of the spectroscopic and photometric follow-up of two
field galaxies that were selected as possible stellar counterparts of local
high velocity clouds. Our analysis shows that the two systems are distant (D>20
Mpc) dwarf irregular galaxies unrelated to the local HI clouds. However, the
newly derived distance and structural parameters reveal that the two galaxies
have luminosities and effective radii very similar to the recently identified
Ultra Diffuse Galaxies (UDGs). At odds with classical UDGs, they are remarkably
isolated, having no known giant galaxy within ~2.0 Mpc. Moreover, one of them
has a very high gas content compared to galaxies of similar stellar mass, with
a HI to stellar mass ratio M_HI/M_* ~90, typical of almost-dark dwarfs.
Expanding on this finding, we show that extended dwarf irregulars overlap the
distribution of UDGs in the M_V vs. log(r_e) plane and that the sequence
including dwarf spheroidals, dwarf irregulars and UDGs appears as continuously
populated in this plane.
