  Let $A$ be a finite dimensional algebra over an algebraically closed field
$k$, and $M$ be a partial tilting $A$-module. We prove that the Bongartz
$\tau$-tilting complement of $M$ coincides with its Bongartz complement, and
then we give a new proof of that every almost complete tilting $A$-module has
at most two complements. Let $A=kQ$ be a path algebra. We prove that the
support $\tau$-tilting quiver $\overrightarrow{Q}({\rm s}\tau$-${\rm tilt} A)$
of $A$ is connected. As an application, we investigate the conjecture of Happel
and Unger in [9] which claims that each connected component of the tilting
quiver $\overrightarrow{Q}({\rm tilt} A)$ contains only finitely many
non-saturated vertices. We prove that this conjecture is true for $Q$ being all
Dynkin and Euclidean quivers and wild quivers with two or three vertices, and
we also give an example to indicates that this conjecture is not true if $Q$ is
a wild quiver with four vertices.
