  We have compiled and analysed reports from the literature of
(quasi-)simultaneous observations of X-ray transients at radio and X-ray
wavelengths and compared them with each other and with more unusual
radio-bright sources such as Cygnus X-3, GRS 1915+105 and Circinus X-1. There
exists a significant (>97% likelihood) positive (rank) correlation between the
peak X-ray flux P_X and radio flux density P_R for the black hole candidate
(BHC) systems, and a marginally significant positive (rank) correlation for the
neutron star (NS) systems. This is further evidence for a coupling between
accretion and outflows in X-ray binary systems, in this case implying a
relation between peak disc-accretion-rate and the number of synchroton-emitting
electrons ejected. However, we also show that the distribution of `radio
loudness', P_R/P_X, is significantly different for the two samples, in the
sense that the BHCs generally have a higher ratio of P_R/P_X. The origin of
this discrepancy is uncertain, but probably reflects differences in the
energetics and/or radiative efficiency of flows around the neutron stars and
black holes; we briefly discuss some of these possibilities. We conclude that
these data point to the formation of a mildly relativistic jet whose luminosity
is a function of the accretion rate, in the majority, if not all, of X-ray
transient outbursts, but whose relation to the observed X-ray emission is
dependent on the nature of the accreting compact object. (Abridged).
