  I show that the 17th century eruption of the massive luminous blue variable
(LBV) star P Cygni can be explained by mass transfer to a B-type binary
companion in an eccentric orbit, under the assumption that the luminosity peaks
occurred close to periastron passages. The mass was accreted by the companion
and liberated gravitational energy, part of which went to an increase in
luminosity. I find that mass transfer of ~ 0.1 solar masses to a B-type binary
companion of ~3-6 solar masses can account for the energy of the eruption, and
for the decreasing time interval between the observed peaks in the visual light
curve of the eruption. Such a companion is predicted to have an orbital period
of ~7 years, and its Doppler shift should be possible to detect with high
resolution spectroscopic observations. Explaining the eruption of P Cygni by
mass transfer further supports the conjecture that all major LBV eruptions are
triggered by interaction of an unstable LBV with a stellar companion.
