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

Orbital motion of the young brown dwarf companion TWA 5 B