OAO/MITSuME Photometry of Dwarf Novae. III. CSS130418:174033+414756

We report on multicolour photometry of the short period dwarf nova CSS130418:174033+414756 during the 2013 superoutburst. The system showed an unusually short superhump period with 0.046346(67) d during stage A, which is one of the shortest periods among dwarf novae below the period minimum. We found that the bluest peaks in $g^{\mathrm{\prime }}-I_{\mathrm{c}}$ colour variations tend to coincide with the brightness minima of the superhump modulations. We also studied nightly-averaged superhump amplitudes in $g^{\mathrm{\prime }}$, $R_{\mathrm{c}}$, and $I_{\rm c}$ bands and found that they have less dependence on wavelength. These properties are likely to be in common with dwarf novae exhibiting superhumps. We successfully obtained $g^{\mathrm{\prime }}-R_{\mathrm{c}}$ and $R_{\mathrm{c}}-I_{\mathrm{c}}$ colours during the temporal dip. The colour indices were significantly bluer compared with other dips of WZ Sge-type dwarf novae. By using the period of the growing superhumps, we estimated the mass ratio to be $q$ = 0.077(5), which is much larger than the previous study.