  X-ray emission from stars much more massive than the Sun was discovered only
35 years ago. Such stars drive fast stellar winds where shocks can develop, and
it is commonly assumed that the X-rays emerge from the shock-heated plasma.
Many massive stars additionally pulsate. However, hitherto it was neither
theoretically predicted nor observed that these pulsations would affect their
X-ray emission. Here we report the discovery of pulsating X-rays from the
massive B-type star Xi1 Canis Majoris. This star is a variable of beta Cephei
type and has a strong magnetic field. Our observations with the XMM-Newton
telescope reveal X-ray pulsations with the same period as the fundamental
stellar pulsation. This discovery challenges our understanding of stellar winds
from massive stars, their X-ray emission, and their magnetism.
