  We present the first measurements of surface differential rotation on a
pre-main sequence binary system. Using intensity (Stokes I) and circularly
polarised (Stokes V) timeseries spectra, taken over eleven nights at the
Anglo-Australian Telescope (AAT), we incorporate a solar-like differential
rotation law into the surface imaging process. We find that both components of
the young, 18 Myr, HD 155555 (V824 Ara, G5IV + K0IV) binary system show
significant differential rotation. The equator-pole laptimes as determined from
the intensity spectra are 80 days for the primary star and 163 days for the
secondary. Similarly for the magnetic spectra we obtain equator-pole laptimes
of 44 and 71 days respectively, showing that the shearing timescale of magnetic
regions is approximately half that found for stellar spots. Both components are
therefore found to have rates of differential rotation similar to those of the
same spectral type main sequence single stars. The results for HD 155555 are
therefore in contrast to those found in other, more evolved, binary systems
where negligible or weak differential rotation has been discovered. We discuss
two possible explanations for this; firstly that at the age of HD 155555 binary
tidal forces have not yet had time to suppress differential rotation, secondly
that the weak differential rotation previously observed on evolved binaries is
a consequence of their large convection zone depths. We suggest that the latter
is the more likely solution and show that both temperature and convection zone
depth (from evolutionary models) are good predictors of differential rotation
strength. Finally, we also examine the possible consequences of the measured
differential rotation on the interaction of binary star coronae.
