1 Introduction

The RS CVn stars constitute one of the major classes of stellar system radio sources. The intense and highly variable radio emission is strictly related to magnetic activity and originates from the interaction of mildly relativistic electrons and magnetic fields (gyrosynchrotron emission) on one or both components. Radio observations of this kind of system can, therefore, allow us to directly investigate non-thermal phenomena occurring in stellar coronae.

AR Lacertae (G2IV+K0IV) is a totally eclipsing RS CVn binary with a short, almost 2 day, orbital period (see Table 1 for the parameters of the system). Its orbital characteristics and strong coronal emission ( $L_{\rm X}\approx 10^{31}~{\rm erg~s}^{-1}$) make this source an ideal target for the studies of coronal structure and the geometry of RS CVn close binaries.

  

Table 1: AR Lac parameters.

The AR Lac coronal emission has been widely studied using most of the X-ray satellites launched in the last twenty years. Early observations, carried out by Swank et al. (1981) using the Einstein Solid State Spectrometer, showed two distinct temperature distributions centred at $7\times 10^6$ and 1.5 to $4\times 10^7$ K. On the basis of Einstein IPC observations, Walter et al. (1983) found that low lying coronae ($\sim$ $0.02\,R_\ast$) seem to be associated with both component stars, while a more extended corona ($\sim$$1\,R_\ast$) only with the K subgiant.

 

 

Table 2: Journal of observations.

	VLA			VLBA+VLA
	C Band	K Band	Q Band	X Band	U Band
Nov. 2-3 (UT)	21:00-08:25	20:30-09:25	20:05-09:00	20:05-09:25	21:30-08:37
Nov. 3-4 (UT)	21:00-08:25	20:30-09:25	20:05-09:00	20:05-09:25	21:30-08:37
Number of scans	10	11	11	7	6
Duration of a scan [m]	19	25	27	44	44
Interval between scans [m]	80	80	80	60	60
Duty cycle (cal-sour-cal) [m]	2-15-2	3-8-3	3-5-3	1.5-3-1.5	1.5-3-1.5
Total Time on source [h]	2.42	2.54	2.4	2.75	2.5

EXOSAT observations by White et al. (1990) confirmed the existence of two different coronal regions associated with different temperature distributions. The observations showed evidence of modulations at low energy (<1 keV), which are not present at high energy (>1 keV). On the other hand, Ottmann et al. (1993) observed evidence of the primary minimum in all energy bandpasses of the ROSAT PSPC.

Despite the numerous X-ray observations, only a few have been carried out at radio wavelengths. Owen & Spangler (1977) first studied the spatial structure of the radio corona of AR Lac, on the basis of flux curve considerations, through observations performed at 4885 MHz with 5 VLA antennas. They did not find evidence of eclipses of either component. The same results were found by Doiron & Mutel (1984) on the basis of VLA observations carried out at 1.48 and 4.9 GHz, using all the 27 antennas of the array in B configuration. These results suggest that the size of the emitting region is larger than the whole system.

During a VLA 3-frequency monitoring of AR Lac, extended over an orbital period, Walter et al. (1987) detected the decay of a relatively, large flare. The event was not observed at 20 cm and the radio spectra seem to turn over between 2 and 6 cm. In the present paper we present the results of a multifrequency observing campaign on the binary system AR Lacertae (HD 210334), simultaneously performed with the Very Large Array (VLA) and the Very Long Baseline Array (VLBA) radio interferometers in the autumn of 1997. The observations cover two orbital periods and took place contemporaneously with X-ray observations of the same system carried out with the SAX satellite by Rodonò et al. (1999).