Dec 23 (Reuters) - Activist investor Jana Partners on Thursday disclosed an about 6.6% stake in Mercury Systems (MRCY.O) and said it would push the defense technology firm to consider options for its business, including a potential sale. Jana, which has built a reputation for working collaboratively and behind the scenes with target companies since Barry Rosenstein founded the firm two decades ago, said Mercury"s shares were undervalued. The activist investor said that it plans to have talks with the company"s management regarding the composition of the board and compensation practices. Mercury did not immediately respond to a Reuters request for comment. The company"s shares, which have fallen over 40% this year, rose 9% in afternoon trading on news of Jana"s stake, which was reported earlier on Thursday by Bloomberg News. Mercury develops and manufactures computer hardware and software products for the aerospace and defense industry. The company had a market capitalization of $2.87 billion as of Wednesday"s close.