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Astron. Astrophys. 342, 709-716 (1999)

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1. Introduction

Algol-type binaries are radio sources with 6 cm luminosities between [FORMULA] and [FORMULA] (Umana et al. 1991, 1998). As in the case of RS CVns, the radio emission arises from interaction of midly relativistic electrons with the magnetic field of the active secondary component and the observed radio spectra can be fitted by models of non-homogeneous gyrosynchrotron emitting plasma. In particular, Umana et al. (1993) were able to reproduce the radio spectra of some selected systems by assuming a core-halo morphology for the radio source.

RZ Cas is a bright, well-known Algol-type system, with orbital period of about 1.20 days, at a distance of 73 pcs. The system consists of an early-type primary (A3 V) and a cooler companion (K0 IV), with an orbital separation of 6.86 solar radii. The other relevant parameters of this binary system, taken from Narusawa et al. (1994), are reported in Table 1.


Table 1.

The system shows many peculiarities such as frequent and unpredictable period variations, which are generally interpreted in terms of mass-transfer episodes between the components (Hegedus et al. 1992), but other possible causes have been investigated (Narusawa et al., 1994). The primary minimum of the optical light-curve has been found on some occasions to resemble a partial eclipse, while on other occasions the observed flat-bottomed minimum resembles that produced by a total eclipse. Since several authors (Narusawa et al., 1994; Maxted et al., 1994) have asserted that the system is a partially eclipsing binary (i=[FORMULA].3[FORMULA][FORMULA].1), this transient totality should be ascribed to other effects such as the presence of circumstellar material or to hot and cold spots on the surface of the primary component, as suggested by Olson (1982).

RZ Cas is also a X-ray and radio source. In particular, Drake et al., (1986), Umana et al., (1991), Umana et al., (1993) have pointed out many analogies with the radio behavior of RS CVns, such as the high radio luminosity and the strong variability of the radio flux.

In this paper we present multi-frequency VLA observations of RZ Cas, which were obtained with the aim to investigate the origin of its radio variability. By analyzing the radio spectrum, observed at different orbital phase, it should be possible to discriminate between intrinsic source structure evolution or absorption by intersystem thermal plasma.

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999