  We have observed the active star $\xi$ Boo A (HD 131156A) with high precision
broadband linear polarimetry contemporaneously with circular
spectropolarimetry. We find both signals are modulated by the 6.43 day rotation
period of $\xi$ Boo A. The signals from the two techniques are 0.25 out of
phase, consistent with the broadband linear polarization resulting from
differential saturation of spectral lines in the global transverse magnetic
field. The mean magnitude of the linear polarization signal is ~4 ppm/G but its
structure is complex and the amplitude of the variations suppressed relative to
the longitudinal magnetic field. The result has important implications for
current attempts to detect polarized light from hot Jupiters orbiting active
stars in the combined light of the star and planet. In such work stellar
activity will manifest as noise, both on the time scale of stellar rotation,
and on longer time scales - where changes in activity level will manifest as a
baseline shift between observing runs.

Solar and Stellar Astrophysics