NOVELTY - The reformer-reactor for reforming hydrocarbon-containing educt fluids to hydrogen-containing product fluid, comprises a burner device (120) and a reformer-reaction chamber (110), which are arranged coaxially and thermally coupled. The burner device and/or the reformer-reaction chamber are formed as annular gap. The burner device is arranged within the reformer-reaction chamber. The burner device comprises a burner-reaction-chamber (122) equipped with a catalyst. The reformer-reaction-chamber comprises a catalyst and from which the educt fluid and/or product fluid is flow-througable.    USE - Useful for reforming hydrocarbon-containing educt fluids to hydrogen-containing product fluid.    ADVANTAGE - The reformer-reactor is compact in size and ensures efficient and effective reforming of hydrocarbon-containing educt fluids to hydrogen-containing product fluid.    DETAILED DESCRIPTION - The reformer-reactor for reforming hydrocarbon-containing educt fluids to hydrogen-containing product fluid, comprises a burner device (120) and a reformer-reaction chamber (110), which are arranged coaxially and thermally coupled. The burner device and/or the reformer-reaction chamber are formed as annular gap. The burner device is arranged within the reformer-reaction chamber. The burner device comprises a burner-reaction-chamber (122) equipped with a catalyst. The reformer-reaction-chamber comprises a catalyst and from which the educt fluid and/or product fluid is flow-througable. The reformer-reactor comprises an evaporator-region (130) and a superheater region, in which the educt fluid flows through. The evaporator-region and the super-heater are formed as annular gap. The educt fluid flows through the reformer-reactor in a coil-shaped line. INDEPENDENT CLAIMS are included for:    (1) a reformer-fuel cell system; and    (2) a method for reforming hydrocarbon-containing educt fluids to hydrogen-containing product fluid.    DESCRIPTION OF DRAWING(S) - The drawing shows a reformer-reactor for reforming hydrocarbon-containing educt fluids to hydrogen-containing product fluid:    Educt fluid line; (104)    Reformer-reaction chamber; (110)    Burner device; (120)    Burner-reaction chamber; (122)    Evaporator-region. (130)