The new XK 4.2 convertible is Jaguar's best opportunity to add some excitement to its slow-selling lineup. The cabrio premiered at Detroit, and it will go on sale in April--at the same time as the coupe. The convertible, which typically comprises the vast majority of XK sales in the United States, shares the coupe's 300-hp, 4.2-liter V-8, paddle-shifted six-speed manu-matic, and aluminum construction. The XK uses a traditional canvas top--rather than a retractable hard top--but in the down position, the folded roof hides neatly beneath a hinged aluminum panel. The XK convertible will start at about $81,500--which is six grand more than the coupe.