  The origin of dust grains in the interstellar medium is still open problem.
\cite{Nicholls2013} found the presence of a significant amount of dust around
V1309 Sco which maybe originate from the merger of a contact binary. We
investigate the origin of dust around V1309 Sco, and suggest that these dust
grains are efficiently produced in the binary-merger ejecta. By means of
\emph{AGBDUST} code, we estimate that $\sim 5.2\times10^{-4} M_\odot$ of dust
grains are produced, and their radii are $\sim 10^{-5}$ cm. These dust grains
mainly are composed of silicate and iron grains. Because the mass of the
binary-merger ejecta is very small, the contribution of dust produced by
binary-merger ejecta to the overall dust production in the interstellar medium
is negligible. However, it is the most important that the discovery of a
significant amount of dust around V1309 Sco offers a direct support for the
idea---common-envelope ejecta provides an ideal environment for dust formation
and growth. Therefore, we confirm that common-envelope ejecta can be important
source of cosmic dust.
