  The emission from young stellar objects (YSOs) in the mid-IR is dominated by
the inner rim of their circumstellar disks. We present an IR-monitoring survey
of about 800 objects in the direction of the Lynds 1688 (L1688) star forming
region over four visibility windows spanning 1.6 years using the \emph{Spitzer}
space telescope in its warm mission phase. Among all lightcurves, 57 sources
are cluster members identified based on their spectral-energy distribution and
X-ray emission. Almost all cluster members show significant variability. The
amplitude of the variability is larger in more embedded YSOs. Ten out of 57
cluster members have periodic variations in the lightcurves with periods
typically between three and seven days, but even for those sources, significant
variability in addition to the periodic signal can be seen. No period is stable
over 1.6 years. Non-periodic lightcurves often still show a preferred timescale
of variability which is longer for more embedded sources. About half of all
sources exhibit redder colors in a fainter state. This is compatible with
time-variable absorption towards the YSO. The other half becomes bluer when
fainter. These colors can only be explained with significant changes in the
structure of the inner disk. No relation between mid-IR variability and stellar
effective temperature or X-ray spectrum is found.

Solar and Stellar Astrophysics