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We present He-like line ratios (resonance, intercombination and forbiddenlines) for totally and partially photoionized media. For solar plasmas, theseline ratios are already widely used for density and temperature diagnostics ofcoronal (collisional) plasmas. In the case of totally and partiallyphotoionized plasmas, He-like line ratios allow for the determination of theionization processes involved in the plasma (photoionization with or without anadditional collisional ionization process), as well as the density and theelectronic temperature. With the new generation of X-ray satellites,Chandra/AXAF, XMM and Astro-E, it will be feasible to obtain both high spectralresolution and high sensitivity observations. Thus in the coming years, theratios of these three components will be measurable for a large number ofnon-solar objects. In particular, these ratios could be applied to the WarmAbsorber-Emitter, commonly present in Active Galactic Nuclei (AGN). A betterunderstanding of the Warm Absorber connection to other regions (Broad LineRegion, Narrow Line Region) in AGN (Seyferts type-1 and type-2, low- andhigh-redshift quasars...) will be an important key to obtaining strongconstraints on unified schemes.  We have calculated He-like line ratios, for Z=6, 7, 8, 10, 12 and 14, takinginto account the upper level radiative cascades which we have computed forradiative and dielectronic recombinations and collisional excitation. Theatomic data are tabulated over a wide range of temperatures in order to be usedfor interpreting a large variety of astrophysical plasmas.Comment: 23 pages, 11 figures, 14 tables. A&AS, in pres

X-ray photoionized plasma diagnostics with helium-like ions. Application to warm absorber-emitter in active galactic nuclei