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One of the main problems in modeling the ionised outflows in Active GalacticNuclei is the unknown distance of the outflowing wind to the central source.Only if the density is known this distance can be determined through theionisation parameter. Here we study density diagnostics based upon O Vtransitions. O V is known to have metastable levels that are density dependent.We study the population of those levels under photoionisation equilibriumconditions and determine for which parameter range they can have a significantpopulation. We find that resonance line trapping plays an important role inreducing the critical densities above which the metastable population becomesimportant. We investigate the K-shell absorption lines from these metastablelevels. Provided that there is a sufficient population of the metastablelevels, the corresponding K-shell absorption lines are detectable and are wellseparated from the main absorption line originating from the ground state. Wethen present the Chandra LETGS spectrum of the Seyfert 1 galaxy Mrk 279 thatmay show for the first time the presence of these metastable level absorptionlines. A firm identification is not yet possible due to both uncertainties inthe observed wavelength of the strongest line as well as uncertainties in thepredicted wavelength. If the line is indeed due to absorption from O V, then wededuce a distance to the central source of one light week to a few lightmonths, depending upon the importance of additional heating processes.Comment: 10 pages, 8 figures, submitted to Astronomy & Astrophysics, main  journa

X-ray/UV campaign on the Mrk 279 outflow: Density diagnostics in Active Galactic Nuclei using O v K-shell absorption lines