Letter to the Editor Periodic changes of veiling and circumstellar grey extinction in DF Tauri I. Dust clouds spiraling into a T Tauri star?

We present 110 simultaneous BV I photometric measurements and low resolution (≈ 400) spectra obtained during 17 consecutive days on the Classical T Tauri star DF Tau. Applying a new method for veiling extraction from low resolution spectra, we derive veiling curves in the B and V photometric bands which closely resemble their corresponding light curves. Analysis of the combined light and veiling curves allows to disentangle the stellar and excess fluxes. The BV stellar fluxes derivation is validated by the I photometric curve which is only slightly contaminated by the excess. The stellar flux exhibits grey variations, up to a factor 2, between the B and I bands. In the first 12 days, these variations are periodic with two minima separated by about 6 days. Yet from day 13th, both the stellar flux and the veiling steadily increase. The overall picture emerging from these results is that of an optically thick accreting cloud, which partially occults the star. As this cloud, located initially within the corotation volume, spirals into the star, the accretion rate increases and its opacity decreases. We also discuss the stellar rotation period and the detection of flare events of few hours duration.