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A melee attack against a helpless character is at a +4 bonus on the attack roll (equivalent to attacking a prone target). A ranged attack gets no special bonus. A helpless defender can't use any Dexterity bonus to AC. In fact, his Dexterity score is treated as if it were 0 and his Dexterity modifier to AC was -5 (and a rogue can sneak attack him).
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