If I go from f# to d# down (f#-d#) is that a diminished third or augmented second interval? I am composing in minor mode and want to know because augmented intervals are forbidden in part writing.

If I go from F# to D# down (F#-D#) is that a diminished third or augmented second interval?

I am composing in minor mode and want to know because augmented intervals are forbidden in part writing.