Two real estate investment trusts,

Boston Properties

(BXP) - Get Report

and

Kimco Realty

(KIM) - Get Report

, are joining the

S&P 500

to replace existing members that are about to be acquired.

Boston Properties will replace

Cinergy

( CIN) in the S&P 500 after the close of trading March 31, Standard & Poor's said. Cinergy is being bought by

Duke Energy

(DUK) - Get Report

, another member of the index.

Kimco will enter the index when trading closes April 3. Kimco is taking the spot currently held by

Jefferson Pilot

(JP) - Get Report

, which is being acquired by S&P 500 member

Lincoln National

(LNC) - Get Report

.

Companies added to an index like the S&P 500 normally see their shares rise because of the demand from tracking funds, and both Boston Properties and Kimco were climbing in extended trading Tuesday.

Kimco was up $1.70, or 4.4%, to $40.45, and Boston was higher by $3.30, or 3.6%, at $94.60.