
\draw[-Latex]
   ($(A)!\ta!(B)$)
   --
   ($(B)!\ta!(A)$)
   node [right] {$r$};


\draw[dashed,-Latex]
   ($(a)!\ta!(b)$)
   --
   ($(b)!\ta!(a)$)
   node [right] {$\mathbb{R}$};

\filldraw (A) circle (1pt) node [above] {$A$};
\filldraw (B) circle (1pt) node [above] {$B$};
\filldraw (a) circle (1pt) node [below] {$a$};
\filldraw (b) circle (1pt) node [below] {$b$};
