The optical variability observed for the so-called active stars is generally accounted for by the presence of photospheric inhomogeneities, such as cool starspots, whose visibility, modulated by the stellar rotation, produces periodic or quasi-periodic light variation of the order of 0.1-0.2 magnitudes in the V-band (cf. Rodonò 1986, 1992a,b and references therein). Multicolor photometry shows that color variations usually indicate a reddening of the star at minimum luminosity, thus supporting the cool starspot hypothesis. However, anticorrelation of the U-B and B-V color indices with respect to the V-band light curve has been observed for some stars, i.e. V711 Tau, TW Lep and UX Ari (Cutispoto 1992; Rodonò & Cutispoto 1992). The orbital or photometric periods of active stars span from less than one day to several weeks. The photometric waves can undergo noticeable changes over time scales as short as few stellar rotations (cf. Figs. 12, 18 and 23 in Cutispoto 1995); hence, in order to investigate the physical characteristics and evolution of spotted areas and the time scale of activity cycles, active stars must be observed systematically. This continuous monitoring program already started at Catania Astrophysical Observatory in the early Sixties and is carried out by using the 0.5 and 1.0 m telescopes of the European Southern Observatory (ESO, La Silla, Chile), the 0.25 m Automatic Photoelectric Telescope of Franklin & Marshall College on Mt. Hopkins (AZ, U.S.A.) and the 0.8 m Automatic Photoelectric Telescope of Catania Astrophysical Observatory on Mt. Etna (Italy). It is aimed to establish the time-extended database that is essential to investigate fundamental topics such as the evolution of spotted areas and the correlation between inhomogeneities at different atmospheric levels (Rodonò et al. 1987; Pagano et al. 1992, 1993; Pallavicini et al. 1993; Küster et al. 1994). Long-term studies are also needed in order to search for the presence of photospheric solar-like activity cycles (see among others Cutispoto & Rodonò 1992; Budding & Zeilik 1995) and differential rotation (Rodonò 1992a). This paper reports on the results obtained by using the 0.5 m and the 1.0 m ESO telescopes and is organized as follows: details on the equipment and observations and reduction procedures are given in Sect. 2, the results the and discussion on individual stars are presented in Sect. 3.
Figure 1: V-band light curve and colors of
HD 5303 = CF Tuc. Phases are reckoned from the spectroscopic ephemeris
(Budding 1985)
Table 1: Comparison (c) and check (ck) stars used for each program star