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Summary form only given. We have developed a formalism for rendering the (near)resonant interaction of two electromagnetic fields with an inhomogeneously broadened system consisting of three-level /spl Lambda/-atoms confined in an ultrathin cell of thickness 10 /spl mu/m or less. The peculiar features of this model stem from the fact that the free interaction time of atoms with two laser fields is solely determined by the wall-to-wall time of atomic flight. Unusual absorptive and dispersive properties of this system have been found. Thus, we show that the absorption spectrum of the probe field and the fluorescence signal from the upper atomic level contain two sub-Doppler dips. One of them is originated from the optical pumping due to the leak of atomic population from the upper atomic level, white the other much narrower dip corresponds to the electromagnetically induced transparency (EIT) effect established in the medium due to the coherent population trapping of the atoms. When the coupling field is tuned into the exact resonance with the corresponding atomic transition, the locations of the two dips on the frequency scale coincide, whereas in the case of nonzero detuning of the coupling field, the two dips are shifted from each other by the amount of this detuning. This is because the EIT is very sensitive to the Raman resonance condition, while the OP is most efficient in the vicinity of the resonance of the probe field with the atomic transition frequency.