Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and the USA (NASA).

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Present address: Columbia Astrophysics Laboratory, Mailcode 5247, 550 West 120th Street, New York, NY 10027, USA.

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...2002)

A safety check with more recent calibration available with SAS 5.3.3 confirms that our results are robust. Note that the C abundance may suffer from some bias since the shape of the EMdistribution below 1 MK cannot be constrained from the XMM-Newton data.

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...1995)

[O/H] = -0.25, [Mg/H] = -0.10, [Si/H] = -0.30, [Ca/H] = -0.20, [Fe/H] = -0.50, [Ni/H] = -0.40.

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...1994)

${A_{\rm C} = 8.13}$ , ${A_{\rm N} = 7.67}$ , ${A_{\rm O} = 8.75}$ , ${A_{\rm Si} = 7.59}$ , ${A_{\rm Ca} = 5.55}$ , ${A_{\rm Fe} = 7.13}$ , ${A_{\rm Ni} = 6.08}$ .

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