  We discuss the Isgur-Wise function $\xi (y)$ in the small velocity (SV) limit
within the QCD sum rule method. The behavior of $\xi (y)$ in the SV limit is
sensitive to the particular form of the duality relations used to decontaminate
the sum rule predictions from the continuum contribution. Peculiarities of the
duality relations in the problem at hand are revealed. It is shown that the
proper requirements of duality and angular isotropy for S wave states lead to
an unambiguous form of the sum rules for the Isgur-Wise function. We illustrate
the constraints due to these requirements using a toy model of the harmonic
oscillator. The slope parameter and the shape of $\xi (y)$ are determined.

High Energy Physics - Phenomenology