Small combining form of hiragana う(u) used to explicitly specify the vowel part of the previous hiragana character, or to show a speaker's pronunciation or extension of that vowel.
どぅ - the sound [du] rather than [do] (ど) or [(d)zu] (づ)
ですぅ - specifies that the final u in です(desu) should not be silent
ですぅぅぅ - specifies that the final u in です(desu) should be very long