The F series is a bond investment made possible by the U.S. Department of Housing and Urban Development, which can safely yield gains of as much as 180% or more over a period of approximately 18-24 months.
They are actually mortgage pass-through certificates often called Agency Debentures. They have been issued by the U.S. Government since 1938. They are exempt from all state and local taxes.
I read about them in the New York times not long ago. They stand to provide anyone who buys them before the middle of December (I can't remember the exact date) some very large gains (like over 100%) in less than 2 years) because the FED is going to reset the rate on these in the next few weeks and that is expected to change their value substantially.