WordNet (r) 3.0 (2006):

billet doux
    n 1: a personal letter to a loved one expressing affection [syn:
         love letter, billet doux]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "billet doux":
   Pastoral Epistle, aerogram, air letter, airgraph, bull,
   chain letter, dead letter, dimissorial, dimissory letter,
   drop letter, encyclical, fan letter, form letter, letter of credit,
   letter of introduction, letters credential, letters of marque,
   letters of request, letters overt, letters patent,
   letters rogatory, love letter, mash note, monitory, newsletter,
   nixie, open letter, pastoral letter, poison-pen letter,
   round robin, valentine