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(Abridged) Main sequence massive stars embedded in an HII region should havethe same chemical abundances as the surrounding nebular gas+dust. The Cocoonnebula, a close-by Galactic HII region ionized by a narrow line B0.5 V singlestar (BD+46 3474), is an ideal target to perform a detailed comparison ofnebular and stellar abundances in the same Galactic HII region. We investigatethe chemical content of O, N and S in the Cocoon nebula from two differentpoints of view: an empirical analysis of the nebular spectrum and a detailedspectroscopic analysis of its ionizing B-type star using state-of-the-artstellar atmosphere modeling. By comparing the stellar and nebular abundances,we aim to indirectly address the long-standing problem of the discrepancy foundbetween abundances obtained from collisionally excited lines (CELs) and opticalrecombination lines in photoionized nebulae. We collect spatially resolvedspectroscopy of the Cocoon nebula and a high resolution optical spectrum of itsionizing star. Standard nebular techniques are used to compute the physicalconditions and gaseous abundances of O, N and S. We perform a self-consistentspectroscopic abundance analysis of BD+46 3474 based on the atmosphere codeFASTWIND to determine the stellar parameters and Si, O, and N abundances. TheCocoon nebula and its ionizing star, located at a distance of 800+-80 pc, havea very similar chemical composition as the Orion nebula and other B-type starsin the solar vicinity. This result agrees with the high degree of homogeneityof the present-day composition of the solar neighbourhood as derived from thestudy of the local cold-gas ISM. The comparison of stellar and nebular CELsabundances in the Cocoon nebula indicates that O and N gas+dust nebular valuesare in better agreement with stellar ones assuming small temperaturefluctuations, of the order of those found in the Orion nebula.

The Cocoon Nebula and its ionizing star: do stellar and nebular abundances agree?