  We report the discovery of a bow-shock-producing star in the vicinity of the
young massive star cluster NGC 3603 using archival data of the Spitzer Space
Telescope. Follow-up optical spectroscopy of this star with Gemini-South led to
its classification as O6 V. The orientation of the bow shock and the distance
to the star (based on its spectral type) suggest that the star was expelled
from the cluster, while the young age of the cluster (~2 Myr) implies that the
ejection was caused by a dynamical few-body encounter in the cluster's core.
The relative position on the sky of the O6 V star and a recently discovered O2
If*/WN6 star (located on the opposite side of NGC 3603) allows us to propose
that both objects were ejected from the cluster via the same dynamical event --
a three-body encounter between a single (O6 V) star and a massive binary (now
the O2 If*/WN6 star). If our proposal is correct, then one can "weigh" the O2
If*/WN6 star using the conservation of the linear momentum. Given a mass of the
O6 V star of 30 Msun, we found that at the moment of ejection the mass of the
O2 If*/WN6 star was 175 Msun. Moreover, the observed X-ray luminosity of the O2
If*/WN6 star (typical of a single star) suggests that the components of this
originally binary system have merged (e.g., because of encounter hardening).
