  Classical T Tauri stars (CTTS) are young, late-type objects, that still
accrete matter from a circumstellar disk. Analytical treatments and numerical
simulations predict instabilities of the accretion shock on the stellar
surface. We search for variability on timescales below a few minutes in the
CTTS TW Hya and AA Tau. TW Hya was observed with SALTICAM on the Southern
African Large Telescope (SALT) in narrow-band filters around the Balmer jump.
The observations were performed in slit mode, which provides a time resolution
of about 0.1 s. For AA Tau we obtained observations with OPTIMA, a single
photon-counting device with even better time resolution. Small-scale
variability typically lasts a few seconds, however, no significant periodicity
is detected. We place a 99 % confidence upper limit on the pulsed fraction of
the lightcurves. The relative amplitude is below 0.001 for TW Hya in the
frequency range 0.02-3 Hz in the 340 nm filter and 0.1-3 Hz in the 380 nm
filter. The corresponding value for AA Tau is an amplitude of 0.005 for 0.02-50
Hz. The relevant timescales indicate that shock instabilites should not be seen
directly in our optical and UV observations, but the predicted oscialltions
would induce observable variations in the reddening. We discuss how the
magnetic field could stabilise the accretion shock.
