2.7 Algols
Algols are eclipsing binaries consisting of a hot, main-sequence primary star of spectral class B to F and
a cool, evolved, less massive secondary. Cool stars in Algols are usually G or K subgiants with outer
convection envelopes which are in contact with their Roche lobes. Tidal interactions force most of these
stars to rotate synchronously with their orbital motions, producing typical rotational velocities between
about 
and 
. The rapid rotation combined with deep convection envelopes produces a
variety of magnetic activity phenomena including starspots in these stars, similar to RS CVn (see
Section 2.4), BY Dra (see Section 2.1), and W UMa binaries (see Section 2.6). Hall (1989
) summarised
observations indicating magnetic activity in Algols. In addition to strong 
emission, X-ray,
and radio emission was detected from a number of Algol binaries which indicated the presence of hot
coronae associated with the cool stars (White and Marshall, 1983; Stewart et al., 1989; Umana
et al., 1989).
Brightness variations due to starspots can be observed in Algols only during primary total eclipses, when
the luminous hot components are hidden. Therefore, because of synchronised rotation, only one hemisphere
of the cool Algol stars can be observed and the photometric data collected are less detailed than
for other spotted binaries. Photospheric continuum variations were nevertheless detected in
cool components and attributed to the effect of cool starspots (Richards, 1990; Olson and
Etzel, 1993; Varricatt, 2000).
