Two real estate investment trusts, Boston Properties ( BXP) and Kimco Realty ( KIM), 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), 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), which is being acquired by S&P 500 member Lincoln National ( LNC). 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.