KDG 218, a Nearby Ultra-Diffuse Galaxy
  We present properties of the low-surface-brightness galaxy KDG 218 observed
with the HST/ACS. The galaxy has a half-light (effective) diameter of $a_e
=47^{\prime\prime}$ and a central surface brightness of $SB_V(0) =
24.4^m/sq^{\prime\prime}$. The galaxy remains unresolved with the HST/ACS,
which implies its distance of {$D > 13.1$ Mpc} and linear effective diameter of
$A_e >3.0$ kpc. We notice that KDG 218 is most likely associated with a galaxy
group around the massive lenticular NGC 4958 galaxy at approximately $22$ Mpc,
or with the Virgo Southern Extension filament at approximately $16.5$ Mpc. At
these distances, the galaxy is classified as an ultra-diffuse galaxy (UDG)
similar to those found in the Virgo, Fornax, and Coma clusters. We also present
a sample of 15 UDG candidates in the Local Volume. These sample galaxies have
the following mean parameters: $\langle D\rangle = 5.1$ Mpc, $\langle
A_e\rangle = 4.8$ kpc, and $\langle SB_B (e) \rangle =
27.4^m/sq^{\prime\prime}$. All the local UDG candidates reside near massive
galaxies located in the regions with the mean stellar mass density (within 1
Mpc) about 50 times greater than the average cosmic density. The local fraction
of UDGs does not exceed 1.5% of the Local Volume population. We notice that the
presented sample of local UDGs is a heterogeneous one containing irregular,
transition, and tidal types, as well as objects consisting of an old stellar
population.
