By the method proposed by Nazarenko (1993) and using observational dat
a obtained by different authors, the physical conditions in the stream
in the neighbourhood of the inner Lagrangian point L(1) were calculat
ed for three interacting binary systems of W Ser-type: beta Lyr, V367
Cyg, RY Sct. It is shown, that as a result of the evolutional expansio
n of the mass losing component the deep layers of its atmosphere reach
the neighbourhood of the first Lagrangian point L(1) and a stream of
axial symmetric shape is formed. Its radius is comparable to the dimen
sion of the mass losing component. The rate of the mass transfer incre
ases up to 10(-5) M./yr The calculated parameters of the stream in the
neighbourhood of the point L(1) explain the main observational proper
ties of the investigated systems (existence of extended circumstellar
matter with a developed structure, high mass transfer rate).