On the Orbital Period Change in the Recurrent Nova U Scorpii

The orbital period of the recurrent nova U Sco has been observed to decrease during the 1999 outburst. In an outburst mass is ejected from the surface of the white dwarf. The separation of the binary system widens and the orbital period increases. We find that magnetic braking between outbursts, mass transfer to the companion, and frictional angular momentum losses during outbursts are all too small to account for this unexpected change. We find, however, that if the secondary has a sufficiently strong magnetic field, B=8x10^3 G, then the ejected material can couple to it and corrotate with the system. The ejected material gains angular momentum while the binary system loses it and the period decreases. If such a strong magnetic field is indeed present, then we predict that a period decrease should be observed also during the current 2010 outburst. If, however, the presence of such a field can be ruled out observationally, then the cause for the period decrease (if confirmed) remains unknown.