  The limiting set of zeros of generalized Bessel polynomials with varying
parameters depending on the degree n cluster in a curve on the complex plane,
which is a finite critical trajectory of a quadratic differential in the form
{\lambda}^2(((z-a)(z-b))/(z^4))dz^2. The motivation of this paper is the
description of the critical graphs of these quadratic differentials. In
particular, we give a necessary and sufficient condition on the existence of
short trajectories.
