When did this happen?  

Avril Lavigne and Nickelback frontman Chad Kroeger's have done a really good job hiding their relationship, few people even knew they were dating?!

Now, after a six-month courtship, the Canadian rockers are engaged to be married!!! Lavigne's rep tells PEOPLE exclusively.

Apparently a romance bloomed as the two became close as they spent time writing together.

 Lavigne, 27, and Kroeger, 37, first got together in February to co-write a song for Lavigne's upcoming fifth studio album.

On Aug. 8, Kroeger popped the question, with a 14-carat diamond sparkler.

It will be Kroeger's first trip down the aisle, and the second marriage for Lavigne, who split from Sum 41 frontman Deryck Whibley in 2009. She had also dated Brody Jenner.