  A plane curve on a the projective space over a field of characteristic zero
is free if its associated sheaf T of tangent vector fields tangent is a free
module. Relatively few free curves are known. Here we prove that a divisor
consisting of a union of curves of a pencil of plane projective curves with the
same degree and with a smooth base locus is a free divisor if and only this
union contains all the singular members of the pencil and its Jacobian ideal is
locally a complete intersection.
