The 28-year-old Duke was an All-Star left-hander for the Pittsburgh Pirates in 2009. He appeared in four games this spring, giving up 25 hits including five home runs in 14 1/3 innings. After pitching for the Arizona Diamondbacks last season, Duke had signed a minor league contract with Houston.
The 32-year-old Cust has 105 major league home runs, 84 of them for the Oakland Athletics between 2007 and 2009. He had an elbow injury this spring and hit 1-for-25 in 11 games.
The Astros have 39 players left in their major league camp, including 18 pitchers.