It looks like U2 have reverted to a positively old-fashioned tactic to promote their new album.

The Dubliners have sent out a number of letters to members of their fan club, featuring a mysterious message hidden under a silhouette of Bono's son and The Edge's daughter.

The text in the letter references William Blake's 'Songs of Innocence and Experience' and a message featuring words that are blacked out reads: “U2 will announce _____ on ______."

It's safe to say that a 'Songs of Experience' announcement is imminent.

See below for pictures of some of the mysterious missives: