NEW YORK (AP) _ American Telephone & Telegraph Co.'s $7.4 billion purchase of NCR Corp. was sealed Thursday, and the computer maker's stock traded for the last time.

AT&T said the merger was finalized when the two companies filed articles of merger with authorities in Maryland, where NCR is incorporated.

Within five business days, NCR shareholders will receive information by mail on how to exchange their stock for AT&T shares, the companies said.

Also, the NCR board of directors redeemed the company's ''poison pill'' plan, which was designed to prevent an unwanted takeover of the Dayton, Ohio- based company. Under the redemption, NCR shareholders of record as of Wednesday will receive notice of the redemption and a payment of 5 cents a share.

In another merger-related transaction, the Capital Group, a mutual-fund and investment manager, completed its purchase of 6.3 million NCR shares Wednesday, the companies said. The shares were automatically converted to 17.9 million AT&T shares under the merger, which valued each NCR share at $110.74.

Thursday marked the last day NCR's stock traded on the New York Stock Exchange. The shares had listed since the 107-year-old company went public in 1926.

NCR shareholders formally approved the takeover at a meeting last week. Under it, NCR will become a subsidiary of the phone company.

AT&T sought NCR to strengthen its weak showing in the computer industry.