1 Introduction

Photospheric inhomogeneities as cool starspots are one of the most typical features of stellar activity. Their visibility, modulated by the stellar rotation, produces periodic or quasi-periodic light variation typically in the 0.1-0.2 magnitudes range (cf. Rodonò 1992a,b and references therein). In most cases multicolor photometry shows a reddening of the star at minimum luminosity (i.e. the light curve amplitude decreases at longer wavelengths), thus supporting the cool starspot hypothesis. However, anticorrelation of the U-B and B-V color indices with respect to the V-band light modulation has been observed for some stars such as V711 Tau, TW Lep, CC Eri and UX Ari (Cutispoto 1992, 1998; Rodonò & Cutispoto 1992; this paper). The orbital/photometric periods of active stars span from less than one day to several weeks and 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; Strassmeier et al. 1997; this paper). 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, is being 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 at Washington Camp. (AZ, U.S.A.) and the 0.8 m Automatic Photoelectric Telescope of Catania Astrophysical Observatory on Mt. Etna (Italy). It is aimed at establishing the time-extended database that is essential to investigate fundamental topics such as the evolution of spotted areas and spot lifetimes, the presence of photospheric solar-like activity cycles and differential rotation (see, among others, Cutispoto & Rodonò 1992; Budding & Zeilik 1995; Rodonò 1992a,b; Lanza et al. 1998), the temporal and/or spatial correlation between inhomogeneities at different atmospheric levels (Pagano et al. 1992, 1993; Pallavicini et al. 1993; Kürster 1996; Kürster et al. 1994, 1997; Catalano et al. 1996; Schmitt et al. 1998). This paper reports on data obtained by using the 0.5 m ESO telescope and is organized as follows: the details on the equipment, observations and reduction procedures are given in Sect. 2, the results and the discussion on individual stars are presented in Sect. 3.