Surface differential rotation of evolved fast rotators

Zeeman-Doppler Imaging enables one to estimate the short term temporal evolution of surface brightness and magnetic structures, under the effect of differential rotation. We present here spectropolarimetric observations secured between 1998 and 2002 for two evolved active stars: the K1 subgiant of the RS CVn system HR 1099 and the single FK Com giant HD 199178. Differential rotation is detected both from brightness and magnetic images, indicating that the rotational shear, roughly solar in magnitude for the single star, is significantly weaker in the binary system. This result suggests that tidal forces, rather than stellar evolution, could be responsible for the lower rotational shear and thus the longer spot lifetime reported for binary systems.