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Aims. CO ro-vibrational lines observed from Herbig group II discs are seen tobe often broad while the same lines observed from group I discs are oftennarrow. This difference is not well understood. In this paper we explore theunderlying cause for this difference and provide a pathway for a betterunderstanding of the geometry and structure of the inner discs around HerbigAe/Be stars. Methods. High-spectral-resolution infrared spectra of COro-vibrational emission from six Herbig Ae/Be candidate stars were taken withCRIRES (CRyogenic high-resolution InfraRed Echelle Spectrograph) at the VLT(Very Large Telescope). From these spectra, we produce individual and co-addedCO ro-vibrational line profiles. We investigate line profile shape differences,and we explore the FWHM (Full Width Half Maximum) variations with J quantumnumber in the context of disc geometry. Furthermore, we put our new sourcesinto the context of earlier observed sources to study a large sample. Forcomparison, we also investigate the FWHM variations with J of modelled COro-vibrational lines from two typical disc geometries produced with thethermo-chemical disc modelling code ProDiMo. Results. - abbreviated - Comparingdust and gas inner disc geometries (inferred by SED and CO ro-vibrationalemission) for the expanded sample of observed Herbig discs, we find no clearcorrespondence between the SED groups of the sources and their inner CO radius.Conclusions. The FWHM versus J is a potential new gas diagnostic for the innerdisc with e.g. a constant FWHM versus J indicating the presence of a large gashole or gap. Both models and observations indicate the potential of this newdiagnostic. - abbreviated -

A proposed new diagnostic for Herbig disc geometry: FWHM versus J of CO ro-vibrational lines