  The inclusive production of omega and phi mesons is studied in the backward
region of the interaction of 12 GeV protons with polyethylene, carbon, and
copper targets. The mesons are measured in e^+ e^- decay channels. The
production cross sections of the mesons are presented as functions of rapidity
y and transverse momentum p_T. The nuclear mass number dependences (A
dependences) are found to be A^{0.710 +/- 0.021(stat) +/- 0.037(syst)} for
omega mesons and A^{0.937 +/- 0.049(stat) +/- 0.018(syst)} for phi mesons in
the region of 0.9 < y < 1.7 and p_T < 0.75 GeV/c.
