Accretion disc evolution in single and binary T Tauri stars

We present theoretical models for the evolution of T Tauri stars surrounded by circumstellar discs. The models include the effects of pre-main-sequenc e stellar and time-dependent disc evolution, and incorporate the effects of stellar magnetic fields acting on the inner disc. For single stars, consis tency with observations in Taurus-Auriga demands that disc dispersal occurs rapidly, on much less than the viscous time-scale of the disc, at roughly the epoch when heating by stellar radiation first dominates over internal v iscous dissipation. Applying the models to close binaries, we find that bec ause the initial conditions for discs in binaries are uncertain, studies of extreme mass ratio systems are required to provide a stringent test of the oretical disc evolution models. We also note that no correlation of the inf rared colours of T Tauri stars with their rotation rate is observed, in app arent contradiction to the predictions of simple magnetospheric accretion m odels.