Aims. We investigate evidence of accretion in a sample of 15 Herbig Ae/Be stars to determine whether these events originate in a remnant gaseous structure from the primordial cloud (rich in hydrogen) or in a metal-rich body (like comets in our Solar System). During such analysis we also determine precise stellar parameters for this sample of stars. Methods. The stars were observed using high resolution spectroscopy (R = 48 000). A synthetic photospheric spectrum was constructed and then

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... ed from the observed one to obtain the circumstellar component. An iterative procedure was applied to find the stellar parameters that were used to build the synthetic photospheric spectrum. Results. Evidence of circumstellar activity were found in four stars: HD 100546, HD 142666, HD 144432 and HD 145718. The presence of redshifted absorption features only in the Balmer lines implies that the accreting material is hydrogen-rich, excluding the possibility that the accretion events might have been created by comet-like bodies. We determined effective temperature, surface gravity, metallicity and the projected rotational velocity for the stars in our sample. Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.