We have investigated the mass accretion rate implied by published surface abundances of Si and C in the white dwarf component of the 3.62 hr period pre-cataclysmic binary and planet host candidate QS Vir ( DA+M2-4 ) .
Diffusion timescales for gravitational settling imply \dot { M } \sim 10 ^ { -16 } M _ { \odot } yr ^ { -1 } for the 1999 epoch of the observations , which is three orders of magnitude lower than measured from a 2006 XMM-Newton observation .
This is the first time that large accretion rate variations have been seen in a detached pre-CV .
A third body in a 14 yr eccentric orbit suggested in a recent eclipse timing study is too distant to perturb the central binary sufficiently to influence accretion .
A hypothetical coronal mass ejection just prior to the XMM-Newton observation might explain the higher accretion rate , but the implied size and frequency of such events appear too great .
We suggest accretion is most likely modulated by a magnetic cycle on the secondary acting as a wind ‘ ‘ accretion switch ’ ’ , a mechanism that can be tested by X-ray and ultraviolet monitoring .
If so , QS Vir and similar pre-CVs could provide powerful insights into hitherto inscrutable cataclysmic variable and M dwarf magnetospheres , and mass and angular momentum loss rates .