Magnetic cycles and photometric variability of T Tauri stars

We consider the effects of stellar magnetic cycles on disc accretion in TTauri stars where the inner region of the disc is disrupted by the stellarfield. Using time-dependent disc models, we show that order unity variations infield strength can strongly modulate the accretion flow for cycle time-scalesof years to decades. Photometric variability is expected to be greatest in theultraviolet, with smaller but still significant variations probable in thenear-infrared. We suggest that variable stellar magnetic fields may contributeto the long-time-scale photometric variability of T Tauri stars. The long-termvariability of T Tauri stars may therefore provide a clue to the nature ofmagnetic dynamo cycles in these stars.Comment: uuencoded compressed postscript file. 8 pages including 3 figures. To appear in MNRAS