  We study the conditions under which a main sequence binary companion to the
central ionizing star of a planetary nebula (PN) might become magnetically
active and thereby display strong X-ray luminosity. Since most PNe are older
than few billion years, any main sequence companion will rotate too slowly to
have magnetic activity and hence bright X-ray emission, unless it is spun-up.
We demonstrate that if the orbital separation during the AGB phase of the PN
progenitor is less than 30-60 AU, main sequence companions in the spectral type
range F7 to M4 will accrete enough angular momentum from the AGB wind to rotate
rapidly, become magnetically active, and exhibit strong X-ray luminosities.
Lower mass M stars and brown dwarfs can also become magnetically active, but
they should have small orbital separations and hence are less likely to survive
the AGB phase of the progenitor. We estimate that 20-30 per cent of elliptical
PNe and 30-50 per cent of bipolar PN are likely to have magnetically active
companions which will reveal themselves in X-ray observations. Re-analysis of
Chandra X-ray Observatory spectroscopy of the compact central source of NGC
7293 indicates that the emitting region of this object possesses abundance
anomalies similar to those of coronally active main-sequence stars.
