VY CMa and VX Sgr observations using APEX SEPIA Band 5. All of the SiO lines originate from the J = 4−3 transition. The arrow in the upper panel marks the stellar systemic velocity. Boxes shaded grey indicate the velocity ranges that can be contaminated by sideband leakage (Sect. 2), with dashed vertical lines indicating the velocity to which peak(s) of emission from the other sideband would ghost, i.e., the dashed lines in the SiO v = 1 panels indicate the velocities at which peaks from the H₂O spectrum could create ghosts. The dashed lines in the H₂O panels indicate the velocities at which peaks from the SiO v = 1J = 4−3 spectrum could create ghosts.