AJC would like to thank T. Sakao and T. Kosugi at the National Observatory of Japan, where most of this work was carried out. The authors also acknowledge PPARC for funding this work, via Rolling Grant no.GR/K59071. AJC also thanks Chris Clarkson of the Astronomy & Astrophysics group for helping with the algebraic reduction. The referee, N. Vilmer, is thanked for her improving and constructive criticism of the paper.
© European Southern Observatory (ESO) 1998
Online publication: March 3, 1998