  We present a simultaneous Chandra and XMM-Newton observation of the Castor
sextett, focusing on Castor A and Castor B, two spectroscopic binaries with
early-type primaries. Of the present day X-ray instruments only Chandra can
isolate the X-ray lightcurves and spectra of A and B. We compare the Chandra
observation with XMM-Newton data obtained simultaneously. Albeit not able to
resolve Castor A and Castor B from each other, the higher sensitivity of
XMM-Newton allows for a quantitative analysis of their combined high-resolution
spectrum. He-like line triplets are used to examine the temperature and the
density in the corona of Castor AB. The temporal variability of Castor AB is
studied using data collected with the European Photon Imaging Camera onboard
XMM-Newton. Strong flare activity is observed, and combining the data acquired
simultaneously with Chandra and XMM-Newton each flare can be assigned to its
host. Our comparison with the conditions of the coronal plasma of other stars
shows that Castor AB behave like typical late-type coronal X-ray emitters
supporting the common notion that the late-type secondaries within each
spectroscopic binary are the sites of the X-ray production.
