AMBER data of BK Vir (data set #3). In each panel, the scaled observed spectrum is plotted by the black solid lines. a)–c) Visibilities observed on the 15.9, 31.8, and 47.8 m baselines (red lines). The corresponding spatial resolutions are also given. d) Uniform-disk diameter (red line) derived by fitting the visibilities shown in panels a)–c). e)–g) Differential phases observed on the 15.9, 31.8, and 47.8 m baselines (red lines). h) Closure phase (red line).