Let A be an integral k-algebra of finite type over an algebraically closed field k of characteristic p > 0. Given a collection D of k-derivations on A, that we interpret as algebraic vector fields on X = Spec(A), we study the group spanned by the hypersurfaces V ( f ) of X invariant under D modulo the rational first integrals of D. We prove that this group is always a finite dimensional F p -vector space, and we give an estimate for its dimension. This is to be related to the results of Jouanolou and others on the number of hypersurfaces invariant under a foliation of codimension 1. As a application, given a k-algebra B between A p and A, we show that the kernel of the pull-back morphism Pic(B) → Pic(A) is a finite F p -vector space. In particular, if A is a UFD, then the Picard group of B is finite.