Yawn contagion is a well-known phenomenon, for example in meetings, but Italian researchers have discovered that the closer you are to someone the more quickly your yawn will follow theirs.
For example in married couples yawns tended to follow in less than a minute. (Slight doubt here for me – how do you know your partner is not actually bored?)
So to be on the safe side if your beloved yawns make sure you do the same ASAP! And if you yawn and your partner doesn’t follow suit, uh-uh!