Must pronominal verbs agree in gender/number?

Question

All verbs that take "être" as their auxiliary verb must agree in gender and number with the subject. This I know. But does this apply also to pronominal verbs?

For instance, "Elle s'est rasée les jambes." Is that correct, or no?

Answer 1

All verbs that take "être" as their auxiliary verb must agree in gender and number with the subject. Does this apply also to pronominal verbs?

Not always.

There are two kinds of pronominal verbs: essentially pronominal verbs and occasionally pronominal verbs. And of course different rules apply to them.

Essentially pronominal verbs

Essentially pronominal verbs are verbs that can only be used with a reflexive pronoun. Furthermore, that pronoun has no other grammatical function than to be the reflexive pronoun of that verb. This means that it is not a complement, direct or indirect, and does not represent the subject or any other part of the sentence. Such verbs include "se souvenir", "se méfier", "s'enquérir", "s'enfuir", "s'absenter", "s'immiscer", etc. Basically, you cannot do those actions to someone or something else.

For this kind of pronominal verbs, past participles do agree in gender and number with the subject. For example:

Marie s'est absentée.

Les filles se sont enfuies.

Les hommes se sont immiscés dans la conversation.

Occasionally pronominal verbs

Occasionally pronominal verbs are verbs that can be used in other instances than reflexively. The reflexive pronoun in these cases can be used as a direct complement, but is not always. Such verbs include "se raser", "s'acheter", "se reconnaître", "se parler", "se couper", "se préparer", "se laver", etc.

For this kind of pronominal verbs, the rule of agreement with the auxiliary "avoir" is used. That is to say that the past participle of those verbs agree with the direct complement if it is placed before the verb.

An easy way to figure out the agreement of the past participle of occasionally pronominal verbs is to replace the reflexive auxiliary "être" with the regular auxiliary "avoir". Let's take your example of "Elle s'est rasé(e) les jambes.":

Elle a rasé quoi? Ses jambes.

The direct complement is placed after the verb and thus the past participle cannot agree with it.

Elle s'est rasé les jambes.

However, if what she had been shaving had not been specified, the case would have been different:

Elle s'est rasé(e).
Elle a rasé quoi? Herself, represented by se (s').

The direct complement is placed before the verb.

Elle s'est rasée.

Other examples

Elles ont conservé les lettres qu'elles se sont écrit(es).
Elles ont écrit quoi? Qu' which is replacing les lettres.

The direct complement is placed before the verb.

Elles ont conservé les lettres qu'elles se sont écrites.

Ils se sont parlé(s).
Ils ont parlé qui ou quoi? There is no direct complement.

Ils se sont parlé.

---

The Banque de dépannage linguistique contains several relevant articles on grammar and other French rules. I used its articles on essentially pronominal verbs and occasionally pronominal verbs to draft this answer. Do note that it is only available in French.

Answer 2

La conjugaison correcte est :

Elle s'est rasé les jambes.

En revanche, on dirait :

Je regarde les jambes qu'elle s'est rasées.

On a un rappel plus complet sur cette page.

edité : plusieurs erreurs dans ma réponse initiale (Merci à Stephane Gimenez pour ses critiques et corrections)