Check out the companies making headlines after the bell on Monday:
T-Mobile — T-Mobile shares rose more than 2% in after-hours trading after Warren Buffett's Berkshire Hathaway disclosed a stake in the mobile networking company. A 13F filing showed the conglomerate bought 2.4 million shares of T-Mobile in the third quarter.
Costco — Shares of the wholesale giant popped more than 2% after the company declared a special cash dividend of $10 per share. CFO Richard Galanti said this is possible thanks to Costco's "strong balance sheet."
SmileDirectClub — SmileDirectClub traded 3.2% higher after the bell on the back of better-than-expected quarterly results. The tele-dentistry company posted a loss of 11 cents per share on revenue of $168.5 million. Analysts polled by Refinitiv expected a loss of 14 cents per share on revenue of $146.2 million.
Baidu — The Chinese internet company saw its stock dip 0.6% even after the release of quarterly results that beat analyst expectations. Baidu reported a profit of CNY20.35 per share, adjusted, on revenue of CNY28.23 billion. Analysts had forecast earnings per share of CNY13.62 on revenue of CNY27.45 billion.
Tesla — Tesla shares rallied more than 9% in after-hours trading after S&P Dow Jones Indices said it would add the electric car maker to the S&P 500, effective Dec. 21.