Magnetic Modulation Beneath the Period Gap: Evidence Through ER UMa Dwarf Nova Supercycles

We analyzed long‐term periodic variations of the supercycle (the interval between two superoutbursts) of four ER UMa‐type dwarf novae (ER UMa, V1159 Ori, DI UMa, and IX Dra). Using multi‐year V‐band and visual (vis) photometry obtained from AAVSO, we constructed light curves, identified the epochs of individual superoutbursts, derived their ephemerides, and constructed O–C diagrams. The O–C residuals follow a structured, repeating pattern (quasi‐periodic) across all four of the above‐mentioned dwarf novae, indicating systematic modulation of the timing of the O–C residuals. Multi‐year cycles were detected using a generalized Lomb‐Scargle periodogram. This long‐term periodic variation in ER UMa systems is consistent with a cycle similar to the solar activity cycle, likely generated by the dynamo effect in the secondary star. This Magnetic activity–driven phases changes in Roche‐lobe filling can explain the enhanced mass‐transfer rates and shortened supercycles in ER UMa systems. The results of magnetic activity in the donor also support the persistence of residual donor magnetic braking below the period gap in cataclysmic variables, challenging evolutionary models in which angular momentum loss (AML) declines abruptly across the fully convective boundary at the upper edge (∼3 h) of the period gap rather than undergoing a smooth transition.