NOVELTY - A fuel cell system (1) includes auxiliary burner (20) reacting air with a fuel into auxiliary burner waste gas during the operation. A bypass air line (24) is connected to fuel cell air line (12) on an inlet side between air delivery device (17) and first heat exchanger (14) and connected to the fuel cell air line on an outlet side between the first heat exchanger and fuel cell (2). An auxiliary waste gas line removes auxiliary burner waste gas from the auxiliary burner. A second heat exchanger (23) couples the auxiliary waste gas line with the bypass air line in a heat-transferring manner.    USE - A fuel cell system for use in motor vehicle. Can also be used in stationary applications, e.g., as an additional supply with current and heat in households or industrial areas.    ADVANTAGE - The use of auxiliary burner can rapidly brought the fuel cell to its operating temperature, shortens heat-up phase, and makes it possible to design the residual gas burner for the operation of the fuel cell at normal rating because the auxiliary burner can be switched off at the end of the cold start. Thus, improved efficiency is obtained for the operation of the fuel cell system at normal rating.    DETAILED DESCRIPTION - A fuel cell system (1) comprises fuel cell (2) generating electric current during operation from anode gas and cathode gas; and residual gas burner (3) reacting anode waste gas with cathode waste gas during the operation. An air delivery device (17) feeds air as a cathode gas to the fuel cell during the operation via a fuel cell air line (12). A waste gas line (13) removes burner waste gas from the residual gas burner. A first heat exchanger (14) couples the waste gas line with the fuel cell air line in a heat-transferring manner. An auxiliary burner (20) reacts air with a fuel into auxiliary burner waste gas during the operation. A bypass air line (24) is connected to the fuel cell air line on an inlet side between the air delivery device and the first heat exchanger and connected to the fuel cell air line on an outlet side between the first heat exchanger and the fuel cell. An auxiliary waste gas line removes auxiliary burner waste gas from the auxiliary burner. A second heat exchanger (23) couples the auxiliary waste gas line with the bypass air line in a heat-transferring manner. INDEPENDENT CLAIMS are included for:    (1) process for operating a fuel cell system; and    (2) modular fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing is a simplified, circuit diagram-like general view of the fuel cell system.    Fuel cell system (1)    Fuel cell (2)    Residual gas burner (3)    Fuel cell air line (12)    Waste gas line (13)    First heat exchanger (14)    Air delivery device (17)    Auxiliary burner (20)    Second heat exchanger (23)    Bypass air line (24)