Emulating the Grand Old Duke of York, having built its stake in Sun Microsystems Inc up to 19%, AT&T Co announced yesterday that it plans to sell its entire holding. The decision no doubt reflects the fact that after acquiring NCR Corp, AT&T will have no further need for Sun’s hardware technology and can get any Sun software it needs on normal commercial terms. Sun has agreed to buy back 5m of the 19.1m shares held by the phone company at $36.25 a share, relecting the current market price, and the balance will be placed with institutional investors by Salomon Brothers Inc. Despite having bought shares representing a 15% stake in Sun at a premium, they are now higher and AT&T will show a $137m overall profit.