  We propose a conjecture that a general member of a bracket-generating family
of rational curves in a complex manifold satisfies the formal principle with
convergence, namely, any formal equivalence between such curves is convergent.
If the normal bundles of the rational curves are positive, the conjecture
follows from the results of Commichau-Grauert and Hirschowitz. We prove the
conjecture for the opposite case when the normal bundles are furthest from
positive vector bundles among bracket-generating families, namely, when the
families of rational curves are of Goursat type. The proof uses natural ODEs
associated to rational curves of Goursat type and corresponding Cartan
connections constructed by Doubrov-Komrakov-Morimoto. As an example, we see
that a general line on a smooth cubic fourfold satisfies the formal principle
with convergence.
