  We propose a new mechanism that produces an orbital period change during a
nova outburst. When the ejected material carries away the specific angular
momentum of the white dwarf, the orbital period increases. A magnetic field on
the surface of the secondary star forces a fraction of the ejected material to
corotate with the star, and hence the binary system. The ejected material thus
takes angular momentum from the binary orbit and the orbital period decreases.
We show that for sufficiently strong magnetic fields on the surface of the
secondary star, the total change to the orbital period could even be negative
during a nova outburst, contrary to previous expectations. Accurate
determinations of pre- and post-outburst orbital periods of recurrent nova
systems could test the new mechanism, in addition to providing meaningful
constraints on otherwise difficult to measure physical quantities. We apply our
mechanism to outbursts of the recurrent nova U Sco.
