An analysis of a new and extensive set of spectroscopic observations of the early-type binary system AR Gas, based also on spectral disentangling, has led to the discovery of the spectrum of the secondary star and of line-profile variations of the B4V primary star. A revised spectroscopic orbit based on these and published data is presented. We also present new photometric data, which have allowed us to improve the solution of the light curve and demonstrate the presence of apsidal motion. Determination of the basic physical parameters of the binary has allowed us to conclude that the system combines the main-sequence primary with an A6V: secondary, possibly an Am star. The consistency of our model of AR Cas is documented by a comparison of model atmosphere flux profiles with the profiles recovered from the spectral disentangling. Analysis of the He I 667.8 nm line indicates that line profile variability is present, and that these variations may be periodic.