  An early-type, massive, short-period ($P_{\rm orb}=2^{\rm d}.310951$)
eclipsing spectroscopic binary DN Cas has been re-visited with new spectral and
photometric data. The masses and radii of the components have been obtained as
$M_1=19.04\pm0.07 M_\odot$, $M_2=13.73\pm 0.05M_\odot$ and $R_1=7.22\pm0.06
R_\odot$, $R_2=5.79\pm0.06R_\odot$, respectively. Both components present
synchronous rotation ($V_{\rm rot1}=160\,$km s$^{-1}$, $V_{\rm rot2}=130\,$km
s$^{-1}$) with their orbit. Orbital period analysis yielded a physically bound
additional component in the system with a minimum mass of $M_3=0.88 M_\odot$
orbiting in an eccentric orbit ($e=0.37\pm0.2$) with an orbital period of
$P_{12}=42\pm9$ yrs. High precision absolute parameters of the system allowed
us to derive a distance to DN Cas as 1.7$\pm$0.2 kpc which locates the system
within the borders of the Cas OB6 association (d=1.8 kpc; Mel'nik & Dambis,
2009). The space velocities and the age of DN Cas are in agreement with those
of Cas OB6. The age of DN Cas ($\tau=$3-5 Myr) is found to be 1-2 Myr older
than the embedded clusters (IC 1795, IC 1805 and IC 1848) in the Cas OB6
association, which implies a sequential star formation in the association.
