  Multi-epoch observations have revealed the variability of pre-main sequence
stars and/or their environment. Moreover, structures in orbital motion around
the central star, resulting from planet-disk interaction, are predicted to
cause temporal variations in the brightness distributions of protoplanetary
disks. Through repeated observations with the Very Large Telescope
Interferometer (VLTI) over nearly two decades, the ESO Archive has become a
treasure chest containing high-resolution multi-epoch near- and mid-infrared
observations of the potential planet-forming regions in protoplanetary disks.
We aim to investigate whether the existing multi-epoch observations provide
evidence for the variability of the brightness distributions of the innermost
few astronomical units of protoplanetary disks and to quantify any variations
detected. We present different approaches to search for evidence of temporal
variations based on multi-epoch observations obtained with the VLTI instruments
PIONIER, AMBER, and MIDI for 68 pre-main sequence stars. For nine objects in
our sample, multi-epoch data obtained using equal baselines are available that
allow us to directly detect variations in the visibilities due to temporally
variable brightness distributions. Significant variations of the near-infrared
visibilities obtained in different epochs with PIONIER and/or AMBER for HD
50138, DX Cha, HD 142527, V856 Sco, HD 163296, and R CrA were found. By
estimating the impact of a small variation of the baseline on the measured
squared visibilities, we are able to compare the data of another 12 pre-main
sequence stars. Thereby, we find evidence for temporal variations of the
brightness distribution of one additional object, AK Sco. Besides the two
binaries DX Cha and AK Sco, HD 50138 and V856 Sco also show signs of
variability caused by variations of asymmetric structures in the brightness
distribution.
