  Characteristic properties of magnetic field effects on spin selective
geminate and bulk electron-hole polaron pair (PP) recombination are analyzed in
detail within the approach based on the stochastic Liouville equation. Simple
expressions for the magnetic field (B) dependence of recombination yield and
rate are derived within two models of relative PP motion: free diffusion and
diffusion in the presence of well (cage). The spin evolution of PPs is
described taking in account the relaxation induced by hyperfine interaction,
anisotropic part of the Zeeman interaction induced, as well as $\Delta
g$-mechanism. A large variety of the $B$-dependences of the recombination yield
$Y(B)$ and rate $K(B)$ is obtained depending on the relative weights of
above-mentioned mechanisms. The proposed general method and derived particular
formulas are shown to be quite useful for the analysis of recent experimental
results.
