OAO/MITSuME Photometry of Dwarf Novae. II. HV Virginis and OT J012059.6+325545

Abstract: We report on multicolor photometry of WZ Sge-type dwarf novae, HV Vir and OT J012059.6+325545 during superoutbursts. These systems show early superhumps with the mean periods of 0.057093(45) d for HV Vir and 0.057147(15) d for OT J012059.6+325545, respectively. The observed early superhumps showed a common feature that the brightness minima correspond to the bluest peaks in color variations, which may be a ubiquitous phenomenon among early superhumps of WZ Sge-type dwarf novae. We confirmed that amplitudes of early superhumps depend on wavelength: amplitudes with longer bandpass filters show larger values. This indicates that the light source of early superhumps is generated at the outer region of the vertically-extended accretion disk. On the other hand, amplitudes of ordinary superhumps are likely to be independent of wavelength. This implies that the superhump light source is geometrically thin. We also examined color variations of ordinary superhumps and found that the bluest peaks in $g'-I_{\rm c}$ tend to coincide with the brightness minima, particularily in stage B superhumps. This may reflect that the pressure effect plays a dominant role during stage B superhumps.