  We use the generalized Lugiato-Lefever model to investigate the phenomenon of
Kerr optical frequency comb generation when group-velocity dispersion is null.
In that case, the first dispersion term that plays a leading role is
third-order dispersion. We show that this term is sufficient to allow for the
existence of both bright and dark solitons. We identify the areas in the
parameter space where both kind of solitons can be excited inside the
resonator. We also unveil a phenomenon of hysteretic switching between these
two types of solitons when the power of the pump laser is cyclically varied.
