  In 1998 we made UV spectroscopic observations with HST/STIS of A0620-00 and
Cen X-4, which are two X-ray novae (aka soft X-ray transients). These binary
systems are similar in all respects except that the former contains a black
hole and the latter contains a neutron star. A UV spectrum (1700-3100A) is
presented for the quiescent state of each system in the context of previously
published UV/optical and X-ray data. The non-stellar, continuum spectrum of
black hole A0620-00 has a prominent UV/optical peak centered at about 3500A. In
contrast the spectrum of neutron-star Cen X-4 lacks a peak and rises steadily
with frequency over the entire UV/optical band. In the optical, the two systems
are comparably luminous. However, black hole A0620-00 is about 6 times less
luminous at 1700A, and about 40 times less luminous in the X-ray band. The
broadband spectrum of A0620-00 is discussed in terms of the advection-dominated
accretion flow model.
