doubled its stake in ADT Ltd. during April and May.

The Boston-based group held the equivalent of 21.64 million ADT shares at May 31, according to a Form 13G filed on Monday with the Securities and Exchange Commission. The stake includes debt securities that are convertible into 3.3 million ADT shares.

Fidelity funds owned 7.35 million shares at March 31, the equivalent of a 4.68 percent stake, according to Technimetrics Inc.

Bermuda-registered ADT provides security systems. Tyco International Inc.

plans to acquire ADT in a transaction originally valued at $5.6 billion, or $29 a share.

Tyco is an Exeter, New Hampshire, manufacturer of disposable medical products and packaging materials.

The Fidelity funds may have purchased ADT shares to get Tyco stock at a discount, according to Tom Burnett, director of research at New York-based Merger Insight.

With Tyco shares closing on Monday at 62 5/8, ADT investors will get to swap each of their shares for $30.14 of Tyco stock. However, ADT shares fell 5/8 today to close at 29 3/8. As a result, investors can buy ADT shares at a discount to the payment Tyco is offering.