  V621 Persei is a detached eclipsing binary in the open cluster chi Persei
which is composed of an early B-type giant star and a main sequence secondary
component. From high-resolution spectroscopic observations and radial
velocities from the literature, we determine the orbital period to be 25.5 days
and the primary velocity semiamplitude to be K = 64.5 +/- 0.4 km/s. No trace of
the secondary star has been found in the spectrum. We solve the discovery light
curves of this totally-eclipsing binary and find that the surface gravity of
the secondary star is log(g_B) = 4.244 +/- 0.054 (cm/s). We compare the
absolute masses and radii of the two stars in the mass--radius diagram, for
different possible values of the primary surface gravity, to the predictions of
stellar models. We find that log(g_A) is approximately 3.55, in agreement with
values found from fitting Balmer lines with synthetic profiles. The expected
masses of the two stars are 12 Msun and 6 Msun, and the expected radii are 10
Rsun and 3 Rsun. The primary component is near the blue loop stage in its
evolution.
