Close binary stars with late spectral-type components, such as RS CVn- or BY Dra-systems, are among the magnetically most active stellar objects known. The magnetic flux generated in the outer convection zone of these rapidly rotating cool stars leads after emergence at the stellar surface to a plethora of activity signatures, which cover large time-, spatial-, and spectral-ranges. Direct observations of coronal magnetic fields are, however, hardly possible and even in the case of the Sun limited to snapshots of localised regions. Investigations focusing on the structure and evolution of large-scale magnetic fields thus frequently apply approximation techniques which are based on the extrapolation of observed magnetic field distributions on the stellar surface into upper atmospheric layers. Here, the potential field source surface approximation technique, which was originally developed for the case of the Sun and later applied to active single stars as well, has been extended to the case of binary systems to investigate their joint magnetospheres and, in particular, the properties of inter-connecting magnetic field structures. The extended modelling technique is described before its capabilities are demonstrated on the basis of a "solar-twin" system, for which synoptic magnetic maps of the Sun observed during different phases of its activity cycle are used. The connectivity of closed coronal loops, 'open' field regions, and inter-connecting field structures is determined for different combinations of solar synoptic maps (e.g. active-active, active-inactive) and compared with the joint magnetospheres of the system V4046Sgr.

J.-F. Donati (University of Toulouse), J. Morin (University of Montpellier)

RS CVn- and BY Dra-systems are among the magnetically most active stellar objects known. The magnetic flux generated in the outer convection zone of these rapidly rotating cool stars leads after emergence at the stellar surface to a plethora of activity signatures detectable in various spectral ranges. Since direct observations of their coronal magnetic fields are hardly possible, investigations frequently approximate the large-scale coronal structure through extrapolations of observed surface magnetic field distributions to the upper atmosphere. Recently, the Potential Field Source Surface approximation technique has been extended to the case of binary stars. Here, this technique is applied to the close BY Dra-system YY Gem to investigate the structure and properties of its joint magnetospheres, in particular, the inter-connecting magnetic field structures. The results are compared with those of the active PMS-system V4046Sgr and of a (hypothetical) "solar-twin" system, for which synoptic magnetic maps of the Sun observed during different phases of its activity cycle are used.

Rapidly rotating, close binary systems with late spectral type components, such as RS CVn and BY Dra systems, typically posses strong magnetic fields which give rise to distinctive magnetic activity signatures. Further more, massive binary stars with early type components have recently been found to harbour significant magnetic fields as well. Since direct observations of coronal magnetic fields are hardly possible, investigations of stellar magnetic activity often make use of extrapolation methods to approximate the magnetosphere of active stars. In this context, the frequently applied Potential Field Source Surface technique has been extended to N-body systems to approximate the joint magnetosphere of close binary systems. Here, the theory of the extrapolation method is described in detail, including the underlying assumptions and limitations. The important aspect of shifting the centres of series expansion by means of multipole translation theorems is visualised, yet a number of equations are unavoidable. The different steps of the extrapolation method are illustrated by examples.