Polls will close at 10pm London time, and the first results are expected at about midnight.
A Brexit vote could have serious repercussions - Yellen.
The Dow gained a further 25 points after Monday's big advance.
Global markets welcome likelihood of a Brexit defeat in Thursday's UK referendum.
Brexit weighs on markets as pro-EU Labour MP is murdered in Leeds.
Wall Street falls in final minutes of trading as Brexit influences Fed.
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