  Disk warps around classical T Tauri stars (CTTS) can periodically obscure the
central star for some viewing geometries. For these so-called AA Tau-like
variables, the obscuring material is located in the inner disk and absorption
spectroscopy allows one to characterize its dust and gas content. Since the
observed emission from CTTS consists of several components (photospheric,
accretion, jet, and disk emission), which can all vary with time, it is
generally challenging to disentangling disk features from emission variability.
Multi-epoch, flux-calibrated, broadband spectra provide us with the necessary
information to cleanly separate absorption from emission variability. We
applied this method to three epochs of VLT/X-Shooter spectra of the CTTS V354
Mon (CSI Mon-660) located in NGC 2264 and find that: (a) the accretion emission
remains virtually unchanged between the three epochs; (b) the broadband flux
evolution is best described by disk material obscuring part of the star, and
(c) the Na and K gas absorption lines show only a minor increase in equivalent
width during phases of high dust extinction. The limits on the absorbing gas
column densities indicate a low gas-to-dust ratio in the inner disk, less than
a tenth of the ISM value. We speculate that the evolutionary state of V354 Mon,
rather old with a low accretion rate, is responsible for the dust excess
through an evolution toward a dust dominated disk or through the fragmentation
of larger bodies that drifted inward from larger radii in a still gas dominated
disk.
