This work is part of a wider
study of the significance of the existence of invariant algebraic curves for
planar polynomial differential systems. The class of real quadratic systems
having a cubic invariant algebraic curve is examined. Using affine canonical
forms for the members of this class we show that no system of this type
has limit cycles except for two cases. For these cases, concrete examples
are given with a limit cycle. We also include a simple and short proof on
the nonexistence of quadratic systems with an algebraic limit cycle of third
degree.