1 Introduction

The MuSiCoS (Multi-Site Continuous Spectroscopy) programme (Catala et al. 1993) is an international collaboration aimed at monitoring the spectral variability of stellar atmospheres on both short and intermediate timescales. This project, gathering about 100 astronomers from 15 countries on five continents, has started to set up a worldwide network of 2 m class telescopes equipped with identical échelle spectrographs. MuSiCoS began in 1990 with the development of a prototype fibre-fed cross-dispersed spectrograph, designed to yield the whole visible spectrum of a star (from 390 to 870 nm) in two exposures, with a mean resolving power of about 35000 (Baudrand & Böhm 1992). This transportable spectrograph has been involved in three MuSiCoS campaigns to date (in 1992, 1994 and 1996) during which it was installed in key sites (Hawaii and South Africa) where such instrumentation was not available. Between these campaigns, the MuSiCoS spectrograph is installed at the 2 m Télescope Bernard Lyot (TBL) at Pic du Midi and available to the entire astronomical community. The MuSiCoS spectrograph has been duplicated twice already and the two copies are currently installed on the 2.5 m Isaac Newton Telescope (La Palma) and the 1.9 m of the South African Astronomical Observatory (Sutherland).

The present paper focuses on the second logical step of the MuSiCoS instrumentation plan: the development of a dedicated Cassegrain polarimetric unit for the MuSiCoS échelle spectrograph. Numerous scientific programmes can indeed benefit from such an instrument and take advantage of the multiplex gain in effective S/N associated with the large spectral domain one can collect in a single exposure. Using "Least-Squares Deconvolution'' (LSD, a new cross-correlation technique devised by Donati et al. 1997), one can for instance try to detect circularly and linearly polarised Zeeman signatures in line profiles of both active solar-type and chemically peculiar stars, in order to obtain information about their surface magnetic field topologies (e.g. Donati & Cameron 1997). One can also study stellar flares and the associated atmospheric particle beam bombardments, by looking for time dependent linear polarisation in several Balmer lines simultaneously (e.g. Saar et al. 1994).

A spectropolarimetric device can also be useful to measure the chromatic variation of continuum linear and circular polarisation from diffusing circumstellar envelopes and/or discs and therefore estimate the size, shape and chemical composition of the diffusing grains, as well as the large-scale geometry and structure of the scattering environment (e.g. Chrysostomou et al. 1997). Similarly, studying how different emission lines (and in particular forbidden lines) are depolarised with respect to the surrounding continuum can provide strong observational constraint on where these lines form with respect to the scattering medium.

This new polarimeter was designed and constructed at Observatoire Midi-Pyrénées in 1996, with CNRS/INSU (Centre National de la Recherche Scientifique, Institut National des Sciences de l'Univers), MENESR (Ministère de l'Éducation Nationale, de l'Enseignement Supérieur et de la Recherche), Région Midi-Pyrénées and OMP (Observatoire Midi-Pyrénées) funds. In this paper, we first present the technical characteristics of this new instrument (see Sect. 2), then describe how it performs in the particular case of the scientific programmes listed above (Sect. 3) and conclude on its potential involvement in future MuSiCoS campaigns.