Mistakes will always be made in French, and now you can learn from them.
Wrong: Ils se sont parlés
Right: Ils se sont parlé
Explanation: You know that in the passé composé and other compound conjugations, French pronominal verbs require agreement with their subjects, as in elles se sont vues (they saw each other) and nous nous sommes connus à Paris (we met each other in Paris). What you may not know is that this only the case when the reflexive pronouns represent direct objects.
Unlike the verbs voir and connaître, which have direct objects, the verb parler needs the preposition à, making the person being talked to an indirect object. This means that the reflexive pronoun in ils se sont parlé (they talked to each other) is indirect, and therefore the past participle does not need agreement. The same is true for se dire (to say to oneself / each other), s'écrire (to write to each other), and other pronominal verbs that need a preposition in French.
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