The Oulipo (Ouvroir de Littérature Potentielle) was founded with the goal of exploring new literary territories. George Perec was one of its most important members. The Oulipo's key concept was the 'constrainte', the constriction, the rule or set of rules that must be applied in the writing of texts. Applying the constrainte of lypogram, Perec wrote his novel La disparition omitting the letter 'e', the most frequent in the French language. The Spanish translation of his novel, El sequestro, was written omitting the letter 'a', the most frequent in Spanish. This article reflects upon the consequences of this double - intra and interlinguistic - translation, and how it affects equivalence between source and target text.