These are unpublished notes in Spanish, written in 1985 when I was finishing my undergraduate degree in Buenos Aires, on defining a group law on degenerate cubics. More precisely, on cubics consisting of a smooth conic and a line. Depending on the number of intersection points of the conic and the line over the given ground field (0,1, or 2 points) you get the circle, the additive or the multiplicative group respectively.