Sun Microsystems Inc has activated the agreement under which it can ask AT&T to buy up to 15% of equity in new shares, asking the telephone giant to buy 879,947 shares for a net total of $40.6m. The sale will give AT&T 2.45% of Sun’s equity and is at a 25% premium to the market price. AT&T is free to top up its holding by buying up to 5% of Sun in the open market for a maximum of 20%. Sun says it wants AT&T’s cash this time to bolster working capital and increase its capital base.