The support of the German Ministry for Research and Technology BMBF and of the Spanish Research Council CICYT is gratefully acknowledged. We thank the Instituto de Astrofisica de Canarias (IAC) for supplying excellent working conditions at La Palma. The ASM/RXTE results are provided by the ASM/RXTE teams at MIT and at the RXTE SOF and GOF at NASA's GSFC. We acknowledge Ron Remillard and Meg Urry for their helpful discussions and advice. We also thank the referee for his helpful remarks.
© European Southern Observatory (ESO) 2000
Online publication: January 18, 2000