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TWO-DIMENSIONAL ELECTRON GAS 2009Interference of fractional microwave induced resistance oscillations withmagneto-intersubband oscillations in a bilayer systemMicrowave induced resistance oscillations (MIRO) whichevolve into “zero resistance states” (ZRS) in high mobilitysamples and for a sufficiently high microwave power arecurrently both under experimental and theoretical investigation.They are governed by the ratio of the radiation frequencyω to the cyclotron frequency ω c = eB/m ∗ , wherem ∗ is the effective mass of the electrons. With increasingmicrowave (MW) power, fractional MIROs (FMIROs)are observed which can be described by ε = ω/ω c = n/mwith integer n and m and m ≥2. If a double quantum well(DQW) is exposed to microwave irradiation, photoresistancediffers from the single-subband case because of additionalintersubband scattering. MIS oscillations are increased/decreasedor show an inversion (peak flip) for lowfrequencies. The oscillating magnetoresistance is caused byan interference of the physical mechanisms responsible forthe MIS oscillations and conventional MIRO, strongly correlatedwith microwave frequency. It has also been foundthat the inelastic mechanism explains photoresistance measurementsin a two-subband system at low temperatures[Wiedmann et al., Phys. Rev. B 78, 121301(R) (2008)].In this contribution we report on experimental and theoreticalstudies of interference between MIS oscillations andFMIROs. In a certain temperature range (8 K