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Astron. Astrophys. 327, 1114-1122 (1997)

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1. Introduction

EV Lac (BD +43o 4305; d=5 pc; Sp=dM4.5e) is a well known flare star, generally regarded as a single star (Pettersen 1980b). From astrometric data, Lippincott (1983) suggested the presence of a substellar mass companion with a period P [FORMULA] 45 y and an orbital eccentricity of 0.5. However, because of the large separation between the two binary components, the EV Lac rotation rate, and consequently its activity level, should not be significantly affected by its binary nature.

EV Lac is also classified as a low-amplitude photometric variable of BY Draconis type. The light curve periodic modulation of BY Dra stars is usually interpreted as evidence of photospheric spotted regions (see, e.g., Rodonò 1986, Rodonò et al. 1986).

Mahmoud and Olah (1981) did not find any periodicity in the B-band light curve of EV Lac over the years 1973 to 1976. However, Pettersen (1980a) reported a photometric period of 4d.373, from data acquired in 1979. This period was later refined to 4d.375 by Pettersen et al. (1983) who analyzed data acquired between 1979 and 1981. The stability of the light-curve ([FORMULA] 0.08 mag) over [FORMULA] 2.5 y led these authors to conclude that the lifetime of the dominant starspot group was longer than 2.5 y.

Subsequently, Pettersen et al. (1992) obtained EV Lac light curves with [FORMULA] 0.1 mag and an almost unchanged phase of light minimum in the years 1979-1989. However, some of these light curves were rather flat, but at a suppressed brightness level, being suggestive of a high degree of spottedness. From these 10 y data, Pettersen et al. (1992) derived a photometric period of 4d.376 and concluded that starspots on EV Lac were only occasionally evenly distributed, being generally located along preferred longitudes.

EV Lac was one of the flare stars selected in the sixties to search for flares and stellar cycles by the IAU Commission 27 Working Group chaired by P. F. Chugainov. Since then, EV Lac has been the primary object of several observational campaigns (e.g.: Cristaldi & Rodonò 1970, 1973, 1975; Mavridis et al. 1982; Pettersen et al. 1983; Gershberg et al. 1991; Alekseev et al. 1994; Abdul-Aziz et al. 1995, Berdyugin et al. 1995) aimed at investigating the statistical properties of optical flares and possible correlations between flare characteristics (such as energetics, time-scales, colours, etc.) and global stellar parameters (such as luminosity, rotation, atmospheric structure, etc.). Previous statistical investigations of M dwarf flares can be found in Lacy et al. (1976), Gershberg & Shakhovskaya (1983) and Shakhovskaya (1989).

Details of possible stellar flare models, flaring behaviour and the scenario of solar-stellar connection can be found in the proceedings of the IAU Coll. 104 on Solar and Stellar Flares (Haisch & Rodonò 1989), and IAU Coll. 151 on Flares and Flashes (Greiner et al. 1995), as well as in reviews by Pettersen (1991), Haisch et al. (1991), and Haisch & Schmitt (1996).

Several authors have investigated whether a correlation between stellar flares and active photospheric areas, as generally seen on the Sun, does exist. Solar flares, in fact, often develop within solar plage areas generally associated with underlying complex sunspot regions. Pettersen et al. (1983) and Roizman (1984) did not find any correlation between the EV Lac flare activity and the rotational modulation due to starspots. These authors, however, investigated only data covering one observation season. On the other hand, from data acquired in 208 h monitoring time in 1976, Andrews (1982) found that flares on EV Lac occurred in groups at 5-6 days interval. Mavridis & Avgoloupis (1986) showed that during the years 1971-1980 there was a 5-year activity cycle in EV Lac. These authors found both that the quiet-state luminosity and the flare activity level were modulated by the same 5 y period, but with a phase shift, in the sense that years of low flare activity were followed by years of high quiet-state luminosity. Doyle (1987) analyzed the data on flare monitoring acquired over the years 1973-1982 and found some correlation between flare frequency and rotation period in 1973-1976. This correlation was not evident in subsequent years.

In this paper we present flare data derived from ten years of EV Lac photometric monitoring carried out at Catania Observatory. The observations and the reduction method we adopted are presented in Sect.  2. The statistical relations among some parameters of the EV Lac flares are given and discussed in Sect.  3.1. The light curves are given in Sect.  3.2. In Sect.  3.3the correlation between flare activity level versus the rotationally modulated light curve phase is investigated. The conclusions are summarized in Sect.  4.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998