  We present simultaneous UBV polarimetric and photometric observations of the
pre-main-sequence binary AK Sco, obtained over 12 nights, slightly less than
the orbital period of 13.6 days. The polarization is a sum of interstellar and
intrinsic polarization, with a significant intrinsic polarization of 1% at
5250A, indicating the presence of circumstellar matter distributed in an
asymmetric geometry. The polarization and its position angle are clearly
variable on time scales of hours and nights, in all 3 wavelengths, with a
behavior related to the orbital motion. The variations have the highest
amplitudes seen so far for pre-main-sequence binaries (~1%, ~30deg) and are
sinusoidal with periods similar to the orbital period and half of it. The
polarization variations are generally correlated with the photometric ones:
when the star gets fainter, it also gets redder and its polarization increases.
The color-magnitude diagram B-V, V exhibits a ratio of total to selective
absorption R=4.3 higher than in normal interstellar clouds (R=3.1). The
interpretation of the simultaneous photometric and polarimetric observations is
that a cloud of circumstellar matter passes in front of the star, decreasing
the amount of direct, unpolarized light, and hence increasing the contribution
of scattered (blue) light. We show that the large amplitude of the polarization
variations can not be reproduced with a single scattering model and axially
symmetric circumbinary or circumstellar disks.
