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NGC288 is a globular cluster with a well-developed blue horizontal branchcovering the u-jump that indicates the onset of diffusion. It is therefore wellsuited to study the effects of diffusion in blue horizontal branch (HB) stars.We compare observed abundances with predictions from stellar evolution modelscalculated with diffusion and from stratified atmospheric models. We verify theeffect of using stratified model spectra to derive atmospheric parameters. Inaddition, we investigate the nature of the overluminous blue HB stars aroundthe u-jump. We defined a new photometric index sz from uvby measurements thatis gravity-sensitive between 8000K and 12000K. Using medium-resolution spectraand Stroemgren photometry, we determined atmospheric parameters (T_eff, log g)and abundances for the blue HB stars. We used both homogeneous and stratifiedmodel spectra for our spectroscopic analyses. The atmospheric parameters andmasses of the hot HB stars in NGC288 show a behaviour seen also in otherclusters for temperatures between 9000K and 14000K. Outside this temperaturerange, however, they instead follow the results found for such stars in omegaCen. The abundances derived from our observations are for most elements (exceptHe and P) within the abundance range expected from evolutionary models thatinclude the effects of atomic diffusion and assume a surface mixed mass of10^-7 M0. The abundances predicted by stratified model atmospheres aregenerally significantly more extreme than observed, except for Mg. Wheneffective temperatures, surface gravities, and masses are determined withstratified model spectra, the hotter stars agree better with canonicalevolutionary predictions. Our results show definite promise towards solving thelong-standing problem of surface gravity and mass discrepancies for hot HBstars, but much work is still needed to arrive at a self-consistent solution.

Hot horizontal branch stars in NGC 288 - effects of diffusion and stratification on their atmospheric parameters