  Non-identical two particle correlation functions probe asymmetries between
the average space-time emission points of different particle species. The
system collective expansion would produce such asymmetry because massive
particles, such as protons, are on average more pushed towards the edge of the
system, than lighter ones, i.e. pions. Measuring pion-kaon, pion-proton and
kaon-proton correlation functions, using the STAR detector, we show that the
data are consistent with the effect of transverse flow in Au-Au collisions at
$\sqrt{s_{NN}} = 130$ GeV.
