Detection of Change in Supercycles in ER Ursae Majoris

We examined data from observations of ER UMa during a period of � 20 years available in the AAVSO, VSNET, AFOEV, NSVS, and VSOLJ databases together with published light curves. The obtained OC diagram revealed a systematic change of the supercycle (time interval between two successive superoutbursts) ranging from 43.6 to 59.2 d. The time-scale of this cycle variation is from 300 to � 1900 d. The number of normal outbursts within the supercycles also varied between 4 and 6, although no strong correlation between this number and the supercycle length was found. We suggest that the appearance of negative superhumps is responsible for the observed varia- tions in the number of normal outbursts. Our results generally confirm the expectations based on the thermal-tidal instability theory.