Left: APEX ground-vibrational ²⁸SiO (black), ²⁹SiO (red), and ³⁰SiO (blue) J = 6−5 spectra toward VY CMa, o Ceti, and W Hya. The intensities of the ²⁹SiO and ³⁰SiO lines were multiplied by two for clarity. Right: Herschel/HIFI ground-vibrational ²⁹SiO (red) and ³⁰SiO (blue) J = 26−25 spectra at around 1.1 THz toward the same sources. The ²⁹SiO emission of VY CMa is blended by the ¹³CO J = 10−9 line from the other sideband. The dashed lines indicate the V_LSR of the sources.