Why does my wtmp only live 24 hours?

[HCART at vax.oxford.ac.uk]

    Every morning when I login on my 3130, the wtmp file
starts afresh with the first user of the day.  The previous wtmp
is not copied to OLDwtmp, which cron should do on the 1st of each
month, but has fallen into a black hole.

    As far as I can see, cron treats wtmp, sulog etc in exactly
the same way, but sulog and the rest survive the night intact,
while wtmp dies each night.  I have accounting turned on.

    Anyone know what might be wiping wtmp once every 24 hours?

Hugh Cartwright.
Physical Chemistry, Oxford.