Augmented sixth chords are a special class of pre–dominant chords with notes that approach the dominant ($\hat{5}$ ) from a half–step below (♯$\hat{4}$ ) and from a half–step above (♭$\hat{6}$ ) simultaneously.
In minor, ♭$\hat{6}$ doesn't require a flat because $\hat{6}$ is lowered in the minor key signature.