Visibilities in four representative CO first overtone lines in the red giant BK Vir (panel a)) and the red supergiant Betelgeuse (panel b)). In both panels the black solid lines represent the scaled observed spectra. The red solid lines represent the visibility obtained for BK Vir on the 47.8 m baseline (panel a)) and that obtained for Betelgeuse on the 11.54 m baseline (panel b)).