Cannondale six13 52cm Carbon top tube?
From my research on your question, it seems that the 52cm Cannondale six13 Road Bikes should have aluminum head tube, seat tube and rear triangle, with the top tube and down tube being carbon fiber. Here's a very good link that describes the Cannondale six13. 

http://www.roadbikereview.com/cat/latest-bikes/road-bike/cannondale/six13/prd_321143_5668crx.aspx