We give necessary and
sufficient conditions for a divisor class on smooth projective anticanonical rational
surfaces to be the class of a smooth rational curve of self-intersection −1. We
characterize smooth projective anticanonical rational surfaces for which the monoid
of classes (modulo algebraic equivalence) of effective divisors is not finitely
generated, extending results of Lahyane for the case of rational surfaces X with
KX2= 0.