YY Gem, finally, is made up of two identical M-dwarfs that orbit each other in only 19 hours. As we look at this system from the side, every 9.5 hours one of the dwarfs is almost completely eclipsed by its companion. Radio astronomy data obtained in 1994 by Manuel Guedel, one of the RGS consortium team members, had shown that both stars were radio sources. So on theoretical grounds, it was believed that both components would show up as X-ray sources. XMM-Newton has confirmed this model very nicely in one EPIC image.
Flares are giant releases of energy that can evolve in a few minutes or over several hours.
A consequence of magnetic instabilities in the hot outer stellar atmospheres, they are well studied on the Sun notably by ESA's SOHO, and on other stars. But the frequency of the flaring on Castor is quite surprising. We are really pleased that so much data on Castor has come out of one single observation. The investigation into the Castor system is to be pursued after the conclusion of XMM-Newton's calibration phase. It promises more exciting insights into this fascinating sextuplet.