1 Introduction

The main sequence stars with infrared excess are believed to be stars with dusty circumstellar disks (Aumann et al. 1984; Backman & Paresce 1993; Vidal-Madjar et al. 1998). These so-called "debris-disks'' must be a by-product of (young?) planetary systems activity like collisions between asteroidal objects (Backman & Paresce 1993). Alternatively, the $\beta\:$ Pictoris disk could be the result of slow evaporation of Chiron-like objects (Lecavelier des Etangs et al. 1996). This disk has a very peculiar status in many ways: it has been extensively imaged in optical (see e.g. Mouillet et al. 1997) and thermal infrared (Lagage & Pantin 1994). The spectroscopic surveys also revealed the presence of very fast spectroscopic variations interpreted by the passage in front of the star of comets in very eccentric orbits (Ferlet et al. 1987; Beust et al. 1991, 1998). In addition, the re-analysis of the photometric measurements done by the Geneva observatory until 1995 revealed that $\beta\:$ Pictoris also presents photometric variations (Lecavelier des Etangs et al. 1995a). Long term variations were observed from 1976 to 1981. Finally a large amplitude ( 0.04 magnitude) and rapid variation was observed in November 1981 during about 10 days. This last variation has been interpreted by an occultation of the star by either a planet or a giant comet (Lecavelier des Etangs et al. 1995a,b, 1997; Lamers et al. 1997).

It thus appears of prime interest to make a photometric survey of $\beta\:$ Pictoris and similar main sequence stars. Such a survey should help to determine the frequency of occurrence of long-term variations among these stars. Moreover, rapid variations similar to the one of November 1981, which could be periodic, must be re-observed to discriminate between the different interpretations, and to find a definite explanation.