A reheat burner includes a channel with a lance projecting thereinto to inject a fuel over an injection plane perpendicular to a channel longitudinal axis. The channel and lance define a vortex generation zone upstream of the injection plane and a mixing zone downstream of the injection plane in the hot gas direction. The mixing zone has a cross section with diverging side walls in the hot gas direction. The diverging side walls define curved surfaces in the hot gas direction having a constant radius.