What is optionally fully convertible debenture?

Optionally fully convertible debenture is an instrument that does not yield interest in the initial period of say, 6 months. After this period option is given to the holder of FCDs to apply for equity at a "premium" for which no additional amourit needs to be paid. The option has to be indicated in the application form itself. However, interest on FCDs is payable at a determined rate from the date of first conversion to the second / final conversion and in lieu of it, equity shares are issued.