  We are carrying out a series of photometric monitoring to measure the
rotation periods of members in the young $\beta$ Pictoris Association, as part
of the RACE-OC project (Rotation and ACtivity Evolution in Open Clusters). In
this paper, we present the results for HD 155555C which is believed to be
physically associated to the spectroscopic binary V824 Ara (HD155555) and thus
constituting a triple system. We collected B, V, and R-band photometric data
timeseries and discovered from periodogram analysis the rotation period P =
4.43d. Combined with stellar radius and projected rotational velocity, we find
this star almost equator-on with an inclination $i$ $\simeq$ 90$^{\circ}$. The
rotational properties of HD155555C fit well into the period distribution of
other $\beta$ Pic members, giving further support to the suggested membership
to the association and to its physical association to V824 Ara. A comparison
with Pre-Main-Sequence isochrones from various models allows us to estimate an
age of 20$\pm$15 Myr for this triple system.
