NEW YORK (Reuters) - Activist investor Blue Harbour's main fund has risen 6.3 percent year to date, boosted in part by a surge in cloud services provider Rackspace Hosting Inc's shares, according to people familiar with the matter.

Rackspace, where Blue Harbour holds a 9.4 percent stake, has seen its stock jump 35 percent since reports surfaced in July that it was in talks to be bought. The company said on Aug. 26 that it agreed to be acquired by Apollo Global Management for $4.3 billion.

Blue Harbour manages $3.3 billion. The firm declined to comment on Wednesday.

(Reporting by Michael Flaherty; Editing by Lisa Von Ahn)