Aberdeen are set to sign out of contract Leyton Orient centre-back Dan Happe. Happe, 27, travelled to the north-east for a medical on Friday and is now expected to be the Dons fourth summer signing – with midfielder Connor Ronan also set to arrive from MLS side Colorado Rapids when the summer transfer window opens on June 15. A 6ft 5in ball-playing left-footer, who can play both sides, Happe would provide competition to Mats Knoester and Gavin Molloy as left-footed centre-half options for Dons boss Jim Goodwin. Happe's signing is a significant boost for Aberdeen, who are looking to strengthen their squad ahead of the new season. The club has been linked with several other signings in recent weeks, but Happe's arrival is the most significant so far. Happe's medical was conducted on Friday, and he is now expected to put pen to paper on a contract with the Dons. The exact terms of the deal have not been disclosed, but it is understood that Happe will join the club on a free transfer. Happe's signing is a major coup for Aberdeen, who are looking to build on their recent success. The club finished third in the Scottish Premiership last season and will be looking to challenge for the title again this year. Happe's arrival will give the club a significant boost, and he is expected to be a key player for the Dons in the coming season.