  We present spectroscopic observations of the massive multiple system
HD\,167971, located in the open cluster NGC\,6604. The brighter component of
the triple system is the overcontact eclipsing binary MY\,Ser with an orbital
period of 3.32\,days. The radial velocities and the previously published UBV
data obtained by \citet{may10} and the UBVRI light curves by \citet{dav88} are
analysed for the physical properties of the components. We determine the
following absolute parameters: for the primary star M$_p$=32.23$\pm$0.54
M$_{\odot}$, R$_p$=14.23$\pm$0.75 R$_{\odot}$; and for the secondary star
M$_s$=30.59$\pm$0.53 M$_{\odot}$, R$_s$=13.89$\pm$0.75 R$_{\odot}$.
Photoelectric times of minimum light are analyzed under the consideration of
the light-time orbit. The center-of-mass of the eclipsing binary is orbiting
around the common center-of-gravity of the triple system with a period of
21.2$\pm$0.7\,yr and with a projected semi-major axis of 5.5$\pm$0.7\,AU. The
mass function for the third star was calculated as 0.370$\pm$0.036 M$_{\odot}$.
The light contributions of the third star to the triple system in the UBV
pass-bands were derived and the intrinsic magnitudes and colors were calculated
individually for the three stars. The components of the eclipsing pair were
classified as O7.5 {\sc iii} and O9.5 {\sc iii}. The intrinsic color indices
for the third star yield a spectral type of (O9.5-B0) {\sc iii-i}. {\bf This
classification leads to constrain the inclination of the third-body orbit,
which should be about 30$^{o}$, and therefore its mass should be about 29
M$_{\odot}$. MY\,Ser is one of the rare massive O-type triple system at a
distance of 1.65$\pm$0.13\,kpc, the same as for the NGC\,6604 embedded in the
Ser\,OB2 association.}
