Under normal circumstances, the fund will invest at least 80% of its assets in, or derive at least 80% of its income from, securities that are exempt from regular federal income tax and may subject the investors to alternative minimum tax. It may invest, without limitation, in securities that may subject the investors to federal alternative minimum tax. The fund invests at least 50% of its portfolio in debt securities rated BBB+ or below or Baa1 or below, or unrated but determined by the fund's investment adviser to be of equivalent quality.