  The starspots on the surface of many chromospherically active binary stars
concentrate on long--lived active longitudes separated by 180 degrees. The
activity shifts between these two longitudes, the "flip-flop" events, have been
observed in single stars like FK Comae and binary stars like $\sigma$
Geminorum. Recently, interferometry has revealed that ellipticity may at least
partly explain the "flip-flop" events in $\sigma$ Geminorum. This idea was
supported by the double peaked shape of the long--term mean light curve of this
star. Here, we show that the long--term mean light curves of fourteen
chromospherically active binaries follow a general model which explains the
connection betweenm orbital motion, starspot distribution changes, ellipticity
and \flip ~events. Surface differential rotation is probably weak in these
stars, because the interference of two constant period waves may explain the
observed light curve changes. These two constant periods are the active
longitude period $(P_{\mathrm{act}})$ and the orbital period
$(P_{\mathrm{orb}})$. We also show how to apply the same model to single stars,
where only the value of $P_{\mathrm{act}}$ is known. Finally, we present a
tentative interference hypothesis about the origin of magnetic fields in all
spectral types of stars.
