  New spectroscopic observations of the double-lined eclipsing binary AQ\,Cas
are presented. All available spectroscopic and photometric observations have
been analysed for the fundamental properties of the components. Analyses show
that the system consists of a massive primary with a mass of 17.63$\pm$0.91
M$_{\odot}$ and radius of 13.48$\pm$0.64R$_{\odot}$ and a secondary with
12.56$\pm$0.81 M$_{\odot}$ and radius of 23.55$\pm$0.73 R$_{\odot}$,
corresponding spectral types of B0.5($\pm$2) II-III + B3($\pm$1) II. The
secondary star fills its corresponding Roche lobe and mass transfer to the
primary star is going on. This stream considerably does affect the photometric
observations both starting from the second quarter up to the first contact of
primary eclipse and just at the second maximum. Thus, the light curve is
distorted and tightly depended on the wavelength of the observations. The
available multi passband light curves have been analysed by taking the stream
effects, as either hot or cool spots, into account. The comparison of the
models and observations in the $\log(L/L_{\odot})$ - $\log T_{eff}$ and $\log g
- \log T_{eff}$ diagrams clearly shows that the more massive star is consistent
with models and is predicted to be close to the phase of hydrogen shell
ignition. Average distance to the system is estimated as 4150$\pm$240 pc using
the BVJHK magnitudes and V-passband extinction.
