S&P 500 to add Western Digital, drop Embarq

CHICAGO, May 21 (Reuters) - Standard & Poor's said on Thursday that Western Digital Corp (WDC.N), a maker of hard drives, will replace Embarq Corp (EQ.N) in its flagship S&P 500 .SPX index after the close of trading on a date to be announced.

Embarq is being acquired by CenturyTel Inc (CTL.N), which is also in the index.

The shares of companies joining the S&P 500 often rise because many investors try to track the index and are required to buy shares of companies that join it.

Western Digital shares rose $1.15 to $25.51 in after-hours trading within minutes of the S&P announcement. They had risen 22 cents in regular trading. The company is based in Lake Forest, California. (Reporting by Jonathan Stempel; Editing by Andre Grenon)