  The partial mixing of matter between the radiative envelope and the
convective core in the early type B--star produces an additional increase of
star luminosity during the main sequence evolution. The high quality data on
stellar mass and luminosity defined from the studies of detached double-lined
eclipsing binaries are used to check the existence of such additional increase.
It is shown that the additional luminosity increase does not contradict to the
observed data of high quality, if the intensity of partial mixing is restricted
by the observed increase in surface helium content.
