Wall Street hangover; Yellen to speak; Nintendo Switch

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1. The hangover: It was fun while it lasted but investors are now dealing with a post-rally hangover.

European and Asian markets were trading in negative territory on Friday and U.S. stock futures were pointing to a weak open.

It's a sharp change in mood for U.S. stocks, which rallied to hit record highs earlier this week after President Trump delivered a well-received address to Congress.

Investors have pushed stocks higher in recent months because of optimism over Trump's plans to cut regulations and slash taxes.

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2. Fed focus: When the head of the U.S. Federal Reserve speaks, investors listen.

Fed chair Janet Yellen is set to give a speech at 1:00 p.m. ET in Chicago, Illinois, to discuss her outlook for the U.S. economy.

Fed vice chairman Stanley Fischer is also giving a speech on Friday about monetary policy in New York. It's due to start at 12:30 p.m.

3. Stocks to watch -- Costco, WPP, Nintendo: Shares in Costco (COST) dropped in extended trading after the retailer issued a disappointing earnings report.

WPP (WPPGY) stock was lower by about 6% in London after the advertising conglomerate reported earnings.

Investors balked at the "nasty combo" of disappointing forecasts for 2017 and a slowdown in the final quarter of 2016, according to Mike van Dulken of Accendo Markets.

But let's switch to something a bit more upbeat: Nintendo Switch!

Shares in Nintendo (NTDOY) jumped by 3.7% in Tokyo after the company released its newest console, which is retailing for $300 in the U.S.

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4. Coming this week:

Friday - Fed chair Yellen speaks at 1 p.m., Nintendo Switch releases worldwide

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