NEW YORK ( TheStreet) -- TripAdvisor ( TRIPV) is joining the benchmark index for U.S. equities.

After Wednesday's close, Standard & Poor's said it's adding the company to the S&P 500, replacing Tellabs ( TLAB), which is being demoted to the S&P Midcap 400 index because its market capitalization has dipped below $1.5 billion.

In turn, Telllabs is bumping Digital River ( DRIV) down to the S&P SmallCap 600 index, and Digital River is displacing Network Equipment Technologies ( NWK), which is the smallest constituent of the S&P SmalllCap 600.

The changes are to take effect at the close of trading on Dec. 20, the same date that TripAdvisor, an online travel company based in Newton, Mass., expected to complete its spinoff from Expedia ( EXPE).

-- Written by Michael Baron in New York.

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