We present a series of spectroscopic and photometric observations spanning up to 13 contiguous nights of three highly veiled T Tauri stars: DG Tau, DR Tau, and DI Cep. DR Tau and DG Tau are so highly veiled that their continuum emission is dominated by the veiling rather than the stellar continuua. All measurements are in agreement with the hypothesis, that the veiling continuum is effectively a pure continuum source. All three stars show strong night-to-night variations in the equivalent widths of the emission lines and smaller variations on shorter time scales. We can conclude from simultaneous photometry and spectroscopy of DG Tau and DI Cep, that the variations in the equivalent widths during each night are mostly due to line flux variations, whereas the night-to-night variations are mostly due to veiling continuum variations, consistent with the observed anti-correlation between the equivalent widths and the veiling strengths in DG Tau and DR Tau (the two quantities are un-correlated in the weakly-veiled star DI Cep). Thus, the timescale for changes in the veiling continuum are of the order of a day whereas the line fluxes vary on timescales of minutes to hours. The short timescale variability of the line fluxes precludes an origin in a quasi-spherical wind. The strengths of all major emission lines in the optical spectrum of DR Tau and DG Tau appear to be quasi-periodic in time, with periods similar to photometric periods reported in the literature. The veiling continuum, however, is much more erratic and not at all suggestive of rotational modulation - a fact which may explains why the veiling-EW correlations are relatively weak. Unfortunately, we do not have enough data to distinguish between line "flares" of an unknown origin or rotational modulation. High resolution spectra suggest that the variations of the line profiles are predominately in the blue wing of the Hα and Hβ profiles in DR Tau, and in the red wing in DI Cep. Lower limits on the area of the Hα emitting regions can be derived from the amount of veiling and the observed line widths by assuming LTE and that the line is optically thick. The areas of the emitting regions must be at least 2-16 times larger than those of the stars. The observed variations of Hα and other strong emission lines lines thus have to be due to changes in the sizes and/or temperatures of the very large emitting regions seen by the observer. The difference between the large emission line and small veiling continuum regions is confirmed by the timescales for line flux and veiling continuum variations: the short-term line flux variations cannot be produced from the site of the veiling continuum production if the later is identified with the accretion region. Interestingly, the same behavior is seen in systems with very strong veiling and quasi-periodic line variations (DG Tau and DR Tau) as well as in a system with moderate to weak veiling and no sign of quasi-periodic variations (DI Cep). These results support the magnetospheric disk accretion model for classical T Tauri stars.
stars: pre-main-sequence - stars: variable - stars: emission-line - stars: individual: DR Tau, DG Tau, DI Cep