  We derive a theoretically allowed domain for the charge radius $\rho$ and
curvature $c$ of the Isgur-Wise function describing the decay $\Lambda_b \to
\Lambda_c l {\bar \nu_l}$. Our method uses crossing symmetry, dispersion
relations and analyticity in the context of the Heavy Quark Effective Theory
but is independent of the specifics of any given model. The experimentally
determined values of the $\Upsilon$ masses have been used as input information.
The results are of interest for testing different models employed to calculate
the heavy baryon formfactors which are used for the extraction of $| V_{cb} |$
from the experimental data.
