All Figures

	Figure 1: Composite EPIC (MOS1+MOS2+PN) image of the  Ori cluster.
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In the text

	Figure 2: RGS 1 and 2 spectra of the central source in the  Ori field. The most intense lines identified are marked. The observed spectrum is produced by the hot star  Ori AB plus some contribution from nearby sources (mainly the flaring source  Ori E) falling in the RGS FOV.
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In the text

Figure 3: Central part ( ) of the composite EPIC image of the  Ori field. Circles mark the sources falling in the RGS FOV with an EPIC flux of that of  Ori AB (all other sources in the FOV contribute less than 10% of the  Ori AB flux); these sources are listed in Table 1 with the corresponding identification numbers. The RGS dispersion direction runs parallel to the PN chip separation (which in this case is almost coincident with the right ascension axis), with wavelength increasing to the left. Thick lines mark the limits of the RGS FOV (5 wide), and thin lines the limits of the extraction region for the RGS spectra.

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In the text

	Figure 4: Close-up view of the central region of the  Ori field during the time interval before the  Ori E flare, showing the position of the components of the  Ori system. Contours are at 2.5, 5, 10, 20, 40 cts/arcsec2. Note the weak source to the north of  Ori E.
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In the text

	Figure 5: EPIC PN light curves of the 4 sources listed in Table 1, binned over 300 s. In the case of  Ori E, the upper axis indicates the photometric phase according to Reiners et al. (2000), with phase 0 indicating the maximum absorption in the U-band (the secondary minimum ends at ). We also show the time intervals used for the time-resolved spectral analysis of  Ori E.
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In the text

	Figure 6: EPIC MOS1 ( blue), MOS2 ( red) and PN ( green) spectra of the 4 sources listed in Table 1. The best-fit models and the residuals are also shown. The spectrum of  Ori AB is dominated by background above 2.5 keV, while the spectrum of  Ori E refers to the total (quiescent + flare) observation. (This figure is available in color in the electronic version.)
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In the text

Figure 7: Selected wavelength intervals of the RGS spectrum of  Ori AB indicating the contamination by other sources in the field. Prominent lines of the main source are identified. The dark solid line represents the RGS spectrum of  Ori E modeled from its EPIC spectrum and shifted according to its position with respect to the main source. Similarly, the dashed line represents the expected RGS spectrum of source #3 (see Table 1) appropriately shifted in wavelength as if it were contaminating the spectrum of the main source (source #4 has almost the same shift and a similar spectrum). Note the unusual strength of the intercombination line of Ne IX at 13.699 Å (see text).

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In the text

	Figure 8: Emission Measure Distribution (EMD) of  Ori AB from the RGS spectrum. Upper panel: EMD (thick line) and contributions of each spectral line weighted by the EMD distribution (thin lines). Lower panel: ratio of observed to predicted lines intensities for different elements.
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In the text

	Figure 9: PN spectra of  Ori E during quiescence and at the peak of the flare. The best-fit model is also shown.
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In the text

	Figure 10: Time evolution of the best-fits parameters and of the X-ray luminosity during the flare on  Ori E.
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In the text