Cat brooch

the Sixth Doctor owned a cat brooch or cat badge, a cat-shaped fashion accessory. He wore it on his left lapel, and touched it on occasion for good luck. (DW: The Twin Dilemma, Attack of the Cybermen)

Behind the scenes

 * One idea for the brooch originally was that it would change its expression to match the mood of the current situation, for example, frightened cats when the Doctor was scared. Eventually this idea was dropped, though it would be subtly implemented in the illustrated webcast Real Time.
 * The production team settled for using different cat brooches. Many fans sent cat brooches to Colin Baker; he has claimed to have worn every single one. (DW: Attack of the Cybermen DVD Commentary)