V. Simon, ON THE RELATION OF CHANGES OF THE PERIOD AND BRIGHTNESS IN THE CLOSE BINARIES SW CYGNI AND U-SAGITTAE, Astronomy and astrophysics, 327(3), 1997, pp. 1087-1093
The analysis of the available data allows to conclude that changes of
the orbital periods in both Algol-type systems, SW Cyg and U Sge, are
correlated with the brightness variations of the late-type loser. The
episodes of decrease of the orbital period are accompanied by minimum
of brightness of the loser in both cases.