  In this paper, we consider Beta$(2-{\alpha},{\alpha})$ (with $1<{\alpha}<2$)
and related ${\Lambda}$-coalescents. If $T^{(n)}$ denotes the length of an
external branch of the $n$-coalescent, we prove the convergence of
$n^{{\alpha}-1}T^{(n)}$ when $n$ tends to $ \infty $, and give the limit. To
this aim, we give asymptotics for the number $\sigma^{(n)}$ of collisions which
occur in the $n$-coalescent until the end of the chosen external branch, and
for the block counting process associated with the $n$-coalescent.
