  With more adaptive optics images available, we aim at detecting orbital
motion for the first time in the system TWA 5 A+B. We measured separation and
position angle between TWA 5 A and B in each high-resolution image available
and followed their change in time, because B should orbit around A. The
astrometric measurement precision is about one milli arc sec. With ten year
difference in epoch, we can clearly detect orbital motion of B around A, a
decrease in separation by ~ 0.0054 arc sec per year and a decrease in position
angle by ~ 0.26 degrees per year. TWA 5 B is a brown dwarf with ~ 25 Jupiter
masses (Neuh\"auser et al. 2000), but having large error bars (4 to 145 Jupiter
masses, Neuh\"auser et al. 2009). Given its large projected separation from the
primary star, ~ 86 AU, and its young age ~ 10 Myrs), it has probably formed
star-like, and would then be a brown dwarf companion. Given the relatively
large changes in separation and position angle between TWA 5 A and B, we can
conclude that they orbit around each other on an eccentric orbit. Some evidence
is found for a curvature in the orbital motion of B around A - most consistent
with an elliptic (e=0.45) orbit. Residuals around the best-fit ellipse are
detected and show a small-amplitude (~ 18 mas) periodic sinusoid with ~ 5.7 yr
period, i.e., fully consistent with the orbit of the inner close pair TWA 5
Aa+b. Measuring these residuals caused by the photocenter wobble - even in
unresolved images - can yield the total mass of the inner pair, so can test
theoretical pre-main sequence models.
