1 Introduction

Flares are frequently observed on magnetically active stars from the radio to the X-ray regime. Sudden reconfiguration of the magnetic field through reconnection is believed to release magnetic energy. In a standard model, coronal electrons are accelerated; part of these electrons collide in the dense chromospheric layer of the stellar atmosphere, producing non-thermal hard X-ray emission (e.g., Dennis 1985). Energy dissipation in the dense layers produces heating of chromospheric material: neutral or already partially ionized material is further ionized. The pressure increases and coronal loops become filled with hot material (chromospheric evaporation; see Antonucci et al. 1984). Radiative cooling occurs mainly through various bound-bound electronic transitions of elements such as Fe, Si, S, Ne, C, N, O, Mg, etc., and continuum emission composed of a superposition of two-photon processes, free-bound emission and thermal bremsstrahlung (free-free emission). The emission lines allow us to derive several plasma properties, such as the distribution of emission measure (EM) as a function of plasma temperature, elemental abundances, or average electron densities (see Mewe 1999). Flares are of central importance to coronal heating. Large flares can display very high temperatures (e.g., Pallavicini & Tagliaferri 1998). Tsuboi et al. (1998) report temperatures of at least 100 MK in a flare on the weak-lined T Tau star V773 Tau. Güdel et al. (1999) showed that the flare EM distribution of the RS CVn binary UX Ari evolved to temperatures of 50 to 100 MK and was accompanied by individual elemental abundance variations. Such variations were already suggested in previous observations, although elemental abundances were often linked together to provide best fits with an "average'' metal abundance (e.g., Ottmann & Schmitt 1996; Favata et al. 2000). Data with low spectral resolution could not disentangle the emission lines from the underlying continuum, which led to some uncertainties about metal-deficient coronal abundances in stars compared to their photospheric values (Schmitt et al. 1996). Furthermore, on some stars, elements with low first ionization potentials (FIP) are found to be enhanced with respect to their photospheric abundances (the FIP effect), while on others they are not (e.g., Drake et al. 1995,1997). HR 1099 (V711 Tauri; HD 22468) is a binary system of the RS CVn class. The system consists of K1 IV + G5 IV stars that are tidally locked with a period of 2.84 days and an inclination of $i=33\hbox{$^\circ$ }$ (Bopp & Fekel 1976; Fekel 1983). At a distance of 28.97 pc (Perryman et al. 1997), it is one of the brightest members of its class. HR 1099 has been extensively studied in the optical, the ultra-violet and the radio, while its extreme ultra-violet (EUV) and X-ray emission was analyzed in the context of surveys of coronal emission from RS CVn systems (Majer et al. 1986; Pasquini et al. 1989; Schmitt et al. 1990; Griffiths & Jordan 1998). Possible detection of rotational modulation in the EUV light curve of HR 1099 was reported by Drake et al. (1994), with a minimum flux occurring near the phase when the G5 star is in front ($\phi =0.5$), consistent with a previously reported correlation between binary phase and X-ray intensity by Agrawal & Vaidya (1988). Recently, Ayres et al. (2001) found in Chandra HETG data of HR 1099 that wavelength shifts in the "Ne X Ly$\alpha$ line are consistent with the orbital motion of the active K1 IV star. In the context of spectroscopy, Drake & Kashyap (1998) studied the coronal metallicity of HR 1099. They reported a coronal iron abundance of $\mathrm{[Fe/H]} \approx -0.4$, consistent with photospheric iron abundance of the K1 star ( $\mathrm{[Fe/H]} \approx -0.6$, Randich et al. 1994). However, noting the difference in photospheric abundances between the primary and the secondary, they argued that the measurements of photospheric abundances of RS CVn systems may be incorrect. Brinkman et al. (2001) analyse XMM-Newton RGS data, discussing coronal elemental abundances in HR 1099. They compare coronal elemental enrichment with solar abundance ratios and find and inverse FIP effect for the time-averaged X-ray emission. The present paper complements the Brinkman et al. paper by investigating the variability of the X-ray emission of HR 1099, the variation of the elemental abundances and of the temperature structure during a large flare (Table 1).

Figure 1: Total 1st and 2nd order RGS2 light curve of HR 1099. Between MJD 51580.702 - 51581.244, RGS1 data were added, because RGS2 was not observing. The ephemeris of Vogt et al. (1999) has been used