Microsoft is trying to drum up sales of its Surface RT tablet by kicking in a free cover. For a limited time starting May 31, Microsoft will throw in a free Touch Cover, Touch Cover Limited Edition, or Type Cover to folks in the U.S. and Canada who buy a Surface RT tablet. Adding a Touch Cover to the purchase normally brings the overall price up by $100, while the cover itself costs $119 separately. The Touch Cover Limited Edition and Type Cover each costs $129 on their own. The Touch Cover is available in cyan, red, magenta, white, or black. The Touch Cover Limited Edition offers a choice of a cyan skull pattern or a magenta flowers pattern. The Type Cover comes in black. The free cover is a limited time offer, according to a Microsoft blog posted Thursday. But the blog failed to reveal just how long it will last. CNET contacted Microsoft to find out when the deal expires and will update the story with more information.