THE South African rand weakened slightly against the U.S. dollar, as markets remained on edge following Russia's invasion of Ukraine. At 1503 GMT, the rand stood at 15.3600 against the dollar, down 0.18% versus its previous close. The rand had weakened more than 2% on Monday to hit 15.4933, as the fallout from tougher sanctions imposed by the West on Russia and the deepening crisis rippled across financial markets and hit risk appetite. A modicum of calm returned to currency markets on Tuesday, after high-level talks between Kyiv and Moscow. But the negotiations ended with no agreement other than to…