  We present multi-epoch, time-resolved optical spectroscopic observations of
the dwarf nova HT Cas, obtained during 1986, 1992, 1995 and 2005 with the aim
to study the properties of emission structures in the system. We determined
that the accretion disc radius, measured from the double-peaked emission line
profiles, is persistently large and lies within the range of 0.45-0.52a, where
a is the binary separation. This is close to the tidal truncation radius
r_max=0.52a. This result contradicts with previous radius measurements. An
extensive set of Doppler maps has revealed a very complex emission structure of
the accretion disc. Apart from a ring of disc emission, the tomograms display
at least three areas of enhanced emission: the hot spot from the area of
interaction between the gas stream and the disc, which is superposed on the
elongated spiral structure, and the extended bright region on the leading side
of the disc, opposite to the location of the hot spot. The position of the hot
spot in all the emission lines is consistent with the trajectory of the gas
stream. However, the peaks of emission are located in the range of distances
0.22-0.30a, which are much closer to the white dwarf than the disc edge. This
suggests that the outer disc regions have a very low density, allowing the gas
stream to flow almost freely before it starts to be seen as an emission source.
We have found that the extended emission region in the leading side of the disc
is always observed at the very edge of the large disc. Observations of other
cataclysmic variables, which show a similar emission structure in their
tomograms, confirm this conclusion. We propose that the leading side bright
region is caused by irradiation of tidally thickened sectors of the outer disc
by the white dwarf and/or hot inner disc regions.
