  Optical spectroscopy of CPD -59 2635, one of the O-type stars in the open
cluster Trumpler 16 in the Carina Nebula, reveals this star to be a
double-lined binary system. We have obtained the first radial velocity orbit
for this system, consisting of a circular solution with a period of 2.2999 days
and semi amplitudes of 208 and 273 km/s. This results in minimum masses of 15
and 11 Msol for the binary components of CPD -59 2635, which we classified as
O8V and O9.5V, though spectral type variations of the order of 1 subclass, that
we identify as the Struve-Sahade effect, seem to be present in both components.
  From ROSAT HRI observations of CPD -59 2635 we determine a luminosity ratio
log(L_x/L_bol)~ -7, which is similar to that observed for other O-type stars in
the Carina Nebula region. No evidence of light variations is present in the
available optical or X-rays data sets.
