  We investigate theoretically the linear conductance of a two-level quantum
dot as a function of the gate voltage and different strength of coupling to the
external electronic system (the reservoir). Apart from the weak coupling
regime, characterized by the Kondo-enhancement of the conductance in the
spinful ground state, a strong coupling regime, which can be called
mixed-valence (MV), is found. This regime is characterized by a qualitative
change of the energy level structure in the dot, resulting in sub- and
super-tunneling coupling of the levels, which in turn yield a novel temperature
dependence of the Kondo effect in the quantum dot.
