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As the third paper of our serial studies that are aim at examining theAGN-host coevolution by using partially obscured AGNs, we extend the broad-linecomposite galaxies (composite AGNs) into ROSAT-selected Seyfert 1.8/1.9galaxies basing upon the RASS/SDSS-DR5 catalog given by Anderson et al.. TheSDSS spectra of in total 92 objects are analyzed by the same method used in ourprevious studies, after requiring the signal-to-noise ratio in the SDSS r' bandis larger than 20. Combing the ROSAT-selected Seyfert galaxies with thecomposite AGNs reinforces the tight correlation between the line ratio[OI]/H\alpha vs. D_n(4000), and establishes a new tight correlation between[SII]/H\alpha vs. D_n(4000). Both correlations suggest the two line ratios areplausible age indicators of the circumnuclear stellar population for typicaltype I AGNs in which the stellar populations are difficult to be derived fromtheir optical spectra. The ROSAT-selected Seyfert galaxies show that the twocorrelations depend on the soft X-ray spectral slope \alpha_X that is roughlyestimated from the hardness ratios by requiring the X-ray count rates within0.1-2.4 keV are larger than 0.02 counts s^-1. However, we fail to establish arelationship between \alpha_X and D_n(4000), which is likely caused by therelatively large uncertainties of both parameters (especially for \alpha_Xbecause of the AGN intrinsic obscuration). The previously establishedL/L_Edd-D_n(4000) evolutionary sequence is reinforced again by the extension tothe ROSAT-selected Seyfert galaxies. These X-ray-selected Seyfert galaxies are,however, biased against the two ends of the sequence, which implies that theX-ray Seyfert galaxies present a population at middle evolutionary stage.

Understanding AGN-host connection in partially obscured active galactic nuclei. Part III: Properties of ROSAT-selected SDSS AGNs