1 Introduction

The class of RS CVn binaries is loosely defined as a class of detached binary systems typically composed of a chromospherically active G or K star, usually evolved, with a late-type main-sequence or subgiant companion (Hall 1976). RS CVn binary systems generally rotate fast with typical orbital periods of a few days; tidal forces (Zahn 1966, 1975) between the two close components have locked their rotation periods to the orbital period ( $P_{\rm orb} = P_{\rm rot}$). These systems display a high level of activity with strong chromospheric line emissions and saturated coronal X-ray emission ( $L_{\rm X}/L_{\rm bol} \approx 10^{-3}$). High-resolution radio observations of RS CVn binary systems show extended magnetospheres (e.g., Mutel et al. 1985), consisting of a diffuse component ("halo'') and an unresolved compact component ("core''). Previous high-resolution data with EUVE showed that the emission measure (EM) distributions of active RS CVn binaries typically display hot plasma with peaks around $\log T_{\rm e} \approx 6.9$ and possibly at 7.35 (Griffiths & Jordan 1998; Griffiths 1999), although the latter peak could not be well constrained. Recent Chandra and XMM-Newton, with more extended spectral ranges, however suggest the presence of a broad EM distribution with local peaks around these temperatures (e.g., Drake et al. 2001; Audard et al. 2001a). Such systems often display powerful energetic flares, with very high temperatures (several tens of MK up to $\approx$100 MK, e.g., Güdel et al. 1999; Franciosini et al. 2001). EM distributions show a very hot (50-100 MK) plasma component that develops and dominates the X-ray emission during flares, although the cooler "quiescent'' part of the EM distribution remains unchanged (e.g., Güdel et al. 1999).

Recent X-ray spectral observations of magnetically active stars have renewed interest in the study of coronal composition. The elemental composition of the solar corona, of the solar wind, and of solar energetic particles (and even of galactic cosmic rays) shows a systematic pattern relative to solar photospheric abundances: elements with a low First Ionization Potential (FIP) are overabundant by factors of 4-6 relative to their respective photospheric abundances, whereas high-FIP elements are of photospheric composition (Meyer 1985; Feldman 1992; Laming et al. 1995). This effect implies the presence of a fractionation mechanism in the chromosphere which has the adequate temperature (5000-10 000 K) to ionize low-FIP elements and leave high-FIP elements mostly in a neutral state. Previous stellar analyses found abundance patterns sometimes at variance with the solar photospheric composition (e.g., Drake et al. 1994; White et al. 1994; Singh et al. 1996; Maggio et al. 1998). High-resolution spectroscopy in the extreme ultraviolet range with EUVE confirmed the overall deficiency of metal abundances (mostly Fe) in active RS CVn binaries (Schmitt et al. 1996a). Detailed studies with EUVE showed either the absence of any FIP-related bias (Drake et al. 1995), or the presence of a FIP effect in inactive stellar coronae (Drake et al. 1997; Laming & Drake 1999). The recent observations of active stars with XMM-Newton and Chandra have shown that an inverse FIP effect may be common in active stars (e.g., Brinkman et al. 2001; Audard et al. 2001a; Drake et al. 2001; Güdel et al. 2001a,b; Huenemoerder et al. 2001). The intermediately active binary star Capella, however, did not show evidence for either a FIP or an inverse FIP effect (Audard et al. 2001b). In contrast, significant increases of metal abundance have been found during flares in M stars, Algol-type and RS CVn-type binaries (e.g., Ottmann & Schmitt 1996; Mewe et al. 1997; Favata et al. 2000); medium-resolution spectra allowed to find complex patterns of abundance increases during a large flare in UX Ari (Güdel et al. 1999), where low-FIP elements increased to higher levels than the high-FIP elements. High-resolution spectroscopy of a flare in HR 1099 with XMM-Newton confirmed this pattern (Audard et al. 2001a). Recently, Güdel et al. (2002) brought up a broader view on the coronal composition in active stars. Their studies of solar-like spectra suggest that a transition from inverse FIP to a normal FIP effect occurs as the activity decreases. Note that their sample of stars have photospheres with abundances similar to the solar composition, hence it avoids any bias due to the uncertainty of the photospheric abundances. In this paper, we show that a similar transition is suggested in RS CVn binary stars; however, these systems are so active that only a transition from an inverse FIP effect to the absence of any FIP bias can be observed.

We present high-quality XMM-Newton data of five RS CVn binary systems with high to intermediate levels of activity: HR 1099 (=V711 Tauri), UX Arietis, $\lambda$ Andromedae, VY Arietis, and Capella (=$\alpha$ Aurigae). Some properties are given in Table 1. These non-eclipsing binaries are X-ray bright (Dempsey et al. 1993), with enhanced activity levels relative to single main-sequence solar-type stars. Despite their binarity, X-ray emission is believed to generally originate from the evolved star (e.g., Ayres et al. 2001; except for Capella, where both giants emit significantly in X-rays, Linsky et al. 1998).

In Sect. 2, we give details on the observations and data reduction; Sect. 3 describes the approach used to interpret the data. In Sect. 4, we discuss the results in terms of a FIP-biased abundance pattern (Sect. 4.1). We investigate the relevance of optical depths in stellar coronae (Sect. 4.2), and the importance of the use of solar (Sect. 4.3) and stellar (Sect. 4.4) photospheric abundances. In Sect. 4.5 we put our results for RS CVn binaries in relation with those in solar analogs. Finally, we present our conclusions in Sect. 5.