Real estate investment companies are paying the smallest premiums to sell bonds in 11 months as office and retail vacancies in America fall and investors seek protection from debt-financed takeovers.

Bonds sold by real estate investment trusts, or REITs, yield an average of 93 basis points more than Treasuries with similar maturities, compared with 99.5 basis points on January 31, according to Merrill Lynch & Company data. REITs outperformed the U.S. corporate bond market this year, rising 0.7%, compared with a 0.4% loss for the broader market.

REITs, including Equity One and HRPT Properties Trust, sold $4.8 billion of bonds, about 25% more than the same period last year. The bonds appeal to investors because office vacancies dropped to 12.5% in the fourth quarter from 14.5% in the year-earlier period, according to real-estate servic es company Cushman & Wakefield.

“Investors can sleep well with REIT bonds, better than any other form of corporate debt,” said Howard Sipzner, chief financial officer of Equity One, a North Miami Beach, Fla.-based REIT with about $2 billion worth of strip malls.

The bonds also are attracting investors because they contain covenants that limit the amount of debt an acquirer can use when taking over the companies, protecting credit ratings and returns.

Covenants typically limit borrowing to 60% of a REIT’s total assets and collateralized debt to 40% of assets. The restrictions often require companies to produce operating income that’s at least 1.5 times interest payments, according to Banc of America Securities LLC. The threat to bondholders increased last year as leveraged buyout groups raised a record $134 billion to buy companies.