The la-type supergiants of spectral type B to G are probably all variable in radial velocity and luminosity; in addition, on the average, the amplitude of the variations increases with increasing intrinsic luminosity and are larger for the earliest and latest spectral types (Abt, 1957; Maeder & Rufener, 1972; Burki et al., 1978; Grenon, 1993). The variations are not strictly periodic, but can be frequently described by a characteristic time Tc, i.e., some kind of ‘most probable periodș (e.g., Sterken, 1977; Rufener et al., 1978; Percy et al., 1979; van Genderen et al., 1992).

It is possible to define a Tc-luminosity-colour relation for the supergiant stars (Maeder & Rufener, 1972; Burki, 1978). However, this relation exhibits a large dispersion, due to the imprecise luminosity and colour determinations for these stars and, essentially, to the fact that each supergiant can show very different Tc values. Good determinations of >Tc require very long-term, continuous, photometric monitorings.