Detection of Change in Supercycles in ER Ursae Majoris

We examined data from observations of ER UMa during a period of similar to 20 years available in the AAVSO, VSNET, AFOEV, NSVS, and VSOLJ databases together with published light curves. The obtained O - C 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 similar 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 variations in the number of normal outbursts. Our results generally confirm the expectations based on the thermal-tidal instability theory.