First massive level of the open RNS string (${\alpha }^{\prime }{M}^2=1,128_B+128_F$)

After the GSO projection, the first massive open-string level has NS oscillator number $3/2$ and Ramond oscillator number $1$. Eight transverse directions give $8+64+\left(\genfrac{}{}{0ex}{}{8}{3}\right)=128$ NS states and $8\cdot 8+8\cdot 8=128$ R states. They form the massive Spin(9) representations $44+84$ and a gamma-traceless vector-spinor of dimension $9\cdot 16-16=128$. The identical chiral oscillator count in a closed string has the different mass normalization ${\alpha }^{\prime }{M}^2=4$.