Relation between the UV slope (β_UV) and the ionizing photon production efficiency (ξ_ion) for stellar population synthesis models with (red) and without (blue) the contribution of nebular emission, i.e., representing the extreme cases of f_esc = 0 and f_esc = 1, respectively. The filled and open circles indicate ξ_ion derived from the SED fitting and from the Hα flux limit assuming f_esc = 0, respectively.