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Magnetic cycles and photometric variability of T Tauri stars

We consider the effects of stellar magnetic cycles on disc accretion in T Tauri stars where the inner region of the disc is disrupted by the stellar field. Using time-dependent disc models, we show that order unity variations in field strength can strongly modulate the accretion flow for cycle time-scales of years to decades. Photometric variability is expected to be greatest in the ultraviolet, with smaller but still significant variations probable in the near-infrared. We suggest that variable stellar magnetic fields may contribute to the long-time-scale photometric variability of T Tauri stars. The long-term variability of T Tauri stars may therefore provide a clue to the nature of magnetic dynamo cycles in these stars.